Infectious Mononucleosis: Lingering Fever and Sore Throat in Young People

Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.

Quite a few young patients come to our fever clinic, such as high school students, university students, and people in their 20s. Among them are sometimes people who say, “I was told it was tonsillitis and got medicine, but after a week my fever still hasn’t gone down.”

“The fever and sore throat have dragged on, and now even the lymph nodes in my neck are swollen. I’m so tired I can’t get up.”

To state the conclusion first, in a young person, symptoms like these may be infectious mononucleosis. It is an infection caused by a common virus, the Epstein-Barr virus, and it can be diagnosed with a blood test, and its treatment and precautions differ from those of strep throat (tonsillitis caused by streptococcus bacteria).

In this article, I roughly explain the features of infectious mononucleosis, how it differs from strep throat and peritonsillar abscess, how long to avoid exercise, and when to see a doctor.

Since October 4, 2026, we also run the fever clinic on Sundays and national holidays
10:00 to 15:00 (no lunch break). No reservation is needed; we accept patients by same-day check-in. PCR test results are given on the same day. On Sundays and national holidays we run only the fever clinic; we do not offer general outpatient care or IV drips. We are closed on Sunday, October 25, and from November we are generally open every Sunday and national holiday. Announcement about the Sunday and national holiday fever clinic

[Conclusion] Infectious mononucleosis is a “long-lasting tonsillitis” from the Epstein-Barr virus, common in young people

The Epstein-Barr virus (EBV, a member of the herpes virus family) is a common virus that most adults have already been infected with[1]. When babies and small children catch it, they often have no symptoms[1].

However, when the first infection happens in adolescence or later, symptoms such as fever, sore throat, and swollen lymph nodes tend to be strong[1]. Its hallmark is swelling of the lymph nodes (small bumps in places such as the neck and armpits that work as part of the immune system), and this is infectious mononucleosis.

It spreads mainly through saliva. Because it can pass on through kissing and sharing drinks, in English it is also called the “kissing disease”[2][1]. The incubation period (the time from infection until symptoms appear) is fairly long, at 4 to 6 weeks[1].

In Japan, too, the age of first infection is getting later

A 2003 explanation from the Japan Institute for Health Security (formerly the National Institute of Infectious Diseases) said that in Japan about 70% are infected by age 2 to 3, and more than 90% have antibodies in their 20s[1]. In Europe and North America, infection in infancy is low, at around 20%, and it has been thought that, to that extent, more young people develop infectious mononucleosis than in Japan[1].

In fact, the situation in Japan is changing too. There is a study that examined stored blood serum from 442 children aged 5 to 7 in Tokyo and nearby prefectures[3]. The rate of positive EBV antibodies was above 80% until the early 1990s, but fell to 59% in 1995 to 1999.

It was estimated to be under 50% in 2006, which shows that the age of first infection keeps getting later[3]. In other words, the number of people who get infectious mononucleosis in their teens and 20s in Japan may keep increasing from now on.

About 1 in 13 people aged 16 to 20 with a sore throat

There is a systematic review by researchers in the United States that pooled 11 studies on the value of examination findings for infectious mononucleosis[4]. A systematic review is a method that gathers studies on the same topic and evaluates them together.

Infectious mononucleosis is most common at ages 5 to 25, and in particular at ages 16 to 20, about 1 in 13 people who came in with a sore throat had this illness[4]. It is mixed in, at a far from rare rate, among young people with “a sore throat and fever.”

Symptoms of infectious mononucleosis: a fever of 38℃ or higher for 1 to 2 weeks, and swelling at the back of the neck

The symptoms look much like ordinary tonsillitis. The differences are “how long” and “how widespread”[1].

There is a study in the United States that followed 150 people with EBV infection confirmed by blood tests from the onset of illness to 1, 2, and 6 months later[5]. The prominent symptoms were sore throat, fatigue, swollen lymph nodes in the neck, and a loss of stamina severe enough to interfere with daily life.

Tests showed an increase in atypical lymphocytes, a kind of white blood cell with a changed shape, and a mild rise in liver function values[5]. Most symptoms and test values were almost back to normal within a month, but fatigue, neck swelling, and throat inflammation improved only slowly[5].

Swollen lymph nodes at the “back of the neck” make it much more likely

The systematic review mentioned earlier shows which findings help with diagnosis using likelihood ratios[4]. A likelihood ratio is a guide to how many times more likely the illness becomes when that finding is present.

Swollen lymph nodes at the back of the neck had a likelihood ratio of 3.1 (95% confidence interval 1.6 to 5.9), pinpoint bleeding spots on the roof of the mouth (the palate) 5.3, and an enlarged spleen 1.9 to 6.6[4]. A 95% confidence interval is the range in which the true value probably lies.

On the other hand, a sore throat and fatigue themselves are present in most patients but are also common in other illnesses, so they cannot be told apart by those alone[4]. “Long for tonsillitis” and “swollen even at the back of the neck” are the entry points for suspecting this illness.

The Ministry of Health, Labour and Welfare’s “Guide to the Appropriate Use of Antimicrobials” also says that with strep throat the lymph nodes at the front of the neck tend to swell, while with infectious mononucleosis the lymph nodes behind the ears and at the back of the neck tend to swell[6].

How it differs from strep throat and peritonsillar abscess [similar to look at, but different illnesses]

When someone has “a sore throat and fever,” it is actually quite hard to tell these apart just by looking at the throat. The Japan Society of Otorhinolaryngology and Head and Neck Surgery also cautions that when the sore throat is severe, not all of it is acute tonsillitis[7].

	Strep throat tonsillitis	Infectious mononucleosis	Peritonsillar abscess
Cause	Bacteria (group A streptococcus)	Virus (Epstein-Barr virus)[1]	Tonsillitis that has worsened, with pus collecting around the tonsil[7]
Features	Sore throat and fever. White pus on the tonsils	Fever for 1 to 2 weeks, swollen lymph nodes in the neck (especially the back), strong fatigue[1]	Severe pain on one side only, difficulty opening the mouth, pain reaching the ear[7]
How it is checked	Rapid throat test	Blood test (white blood cell breakdown, liver function, EBV antibodies)[1]	Looking at the back of the throat at an ear, nose and throat clinic, and imaging if needed
Treatment	An antibiotic (a medicine that kills bacteria), taken to the end of the amount prescribed[6]	Symptomatic treatment (treatment that eases fever and pain)[1]	Draining the pus and an antibiotic[7]
Things to watch for	Do not stop the medicine partway	A rash is likely with amoxicillin[8]. The spleen swells, so avoid strenuous exercise[2]	Strong pain on one side only means the fever clinic or an ear, nose and throat clinic the same day. If you cannot open your mouth or swallow saliva, be seen right away even at night or on a holiday

I explain strep throat tonsillitis in detail in the article on strep throat.

Peritonsillar abscess is characterized by very strong pain on one side only, difficulty opening the mouth, and pain that reaches the ear[7]. Unlike infectious mononucleosis, it needs urgent treatment to drain the pus, so if you notice strong pain on one side only, see the fever clinic or an ear, nose and throat clinic the same day.

If you can barely open your mouth or cannot swallow saliva or fluids, please be seen right away, even at night or on a holiday.

Amoxicillin can cause a rash [which is why diagnosis matters]

For tonsillitis known to be strep throat, the Ministry of Health, Labour and Welfare’s guide also says the basic treatment is to take amoxicillin, a penicillin-type antibiotic, for 10 days[6]. Once strep throat is confirmed, start without hesitation, and it is important to finish all that you are given.

However, it is known that people with infectious mononucleosis who take amoxicillin (a relative of ampicillin) easily develop a rash[1][7]. A report from the 1960s said that a rash appeared in 80 to 100% with ampicillin[8].

There is a study of 238 children admitted to two children’s hospitals in Israel who were diagnosed by EBV blood tests[8]. Of these, 173 had used an antibiotic, and a rash appeared in 32.9%, compared with 23.1% in the children who had not (the difference is not statistically significant).

The 32.9% above is the proportion who developed a rash from any cause during the course of the illness. Among rashes judged to be caused by the antibiotic itself, amoxicillin was the most common of the drugs, at 29.5% (95% confidence interval 18.52 to 42.57)[8]. Although much lower than the old report, this works out to a rash in roughly 1 in 3 to 4 people.

What matters here is that antibiotics themselves are not the villain. For strep throat, finishing the antibiotic is the pillar of treatment[6]. Infectious mononucleosis, on the other hand, is a viral illness, so it is treated with symptomatic care rather than antibiotics[1].

The doctor works out whether it is strep throat or infectious mononucleosis and chooses the medicine. This is the shortest path to making sure you get the treatment you need. If a rash appears after you were told it was tonsillitis and started an antibiotic, do not stop or continue it on your own judgment; contact the medical institution that prescribed it early. Please make a note of the day the rash appeared and the name of the medicine, because they will be important information when choosing medicines in the future.

Diagnosis is by blood test [it cannot be known by looking at the throat]

Whether it is infectious mononucleosis is confirmed with a blood test[1]. Mainly the following 3 things are checked.

In the systematic review mentioned earlier, having 10% or more atypical lymphocytes gave a likelihood ratio of 11.4 (95% confidence interval 2.7 to 35), greatly raising the likelihood[4]. The Ministry of Health, Labour and Welfare’s guide also says that if lymphocytes make up 35% or more on a blood test, the likelihood is higher[6]. In other words, just looking at the blood breakdown gives you quite a good idea.

EBV antibodies appear differently depending on the stage of infection. One blood draw sometimes cannot settle it, and blood may be drawn twice, in the acute phase and in the recovery phase 4 to 6 weeks later, to compare[1].

The simple tests often used abroad (the Paul-Bunnell reaction and the Monospot test) are often negative in Japanese patients and are not widely used[1]. We judge comprehensively, together with the rapid strep test.

[Avoid exercise for 1 month] The spleen swells and, rarely, can rupture

The spleen is an organ deep in the left side of the abdomen, behind the stomach, that processes old blood cells. In infectious mononucleosis the spleen can swell, and sometimes it swells greatly and ruptures (splenic rupture)[1].

A 2023 statement from the American Medical Society for Sports Medicine (AMSSM) says that spleen enlargement is common and rupture is rare, but that it typically occurs within a month of onset[2]. Treatment centers on symptomatic care to ease symptoms[2].

How long to hold back? From studies in the United States and the United Kingdom

There is a study of US military medical data that examined 42 cases from 2006 to 2016 in which splenic injury (the spleen rupturing or being injured) related to infectious mononucleosis was confirmed[9]. Splenic injury came on average 15.4 days after onset, with 73.8% within 21 days and 90.5% within 31 days.

Many of these happened without any injury such as a blow or a fall[9]. In the United States, a 3-week activity restriction had been the guide, but this study proposes extending it to 31 days[9]. Splenic rupture is said to occur in 0.1 to 0.5% of patients with infectious mononucleosis[9].

There is also a systematic review by researchers in the United Kingdom that pooled 85 published case reports of splenic rupture from 1984 to 2014[10]. The average age was 22, 70% were men, and the time from onset to rupture was 14 days on average, and up to 8 weeks in the longest.

Only 14% had a preceding injury, abdominal pain was the most common symptom at 88%, and 9% died[10]. However, these figures come from collecting only the severe cases reported in papers, and are not the proportion among all patients. This review recommends avoiding sports, heavy lifting, and strenuous exercise for 8 weeks[10].

The length of time to hold back differs by country and medical institution. The study of US military medical data proposed up to 31 days from onset, and the UK review proposed an 8-week activity restriction[9][10]. Based on these overseas guides, it is safest to avoid club activities, the gym, contact sports, and heavy lifting for at least 1 month from onset.

Contact sports means sports in which bodies collide, such as rugby, judo, soccer, and basketball. Please check with the doctor who examined you about when to restart. There is also a view that you should first confirm with ultrasound that the spleen swelling has gone down[10].

[Call 119 right away] Signs that suggest splenic rupture, airway obstruction, or encephalitis

If even one of the following applies, call 119 right away instead of going to the fever clinic. This is the same at night and on holidays.

Airway obstruction (blockage of the passage for breathing), meningitis, and encephalitis have been reported, though rarely, as complications of infectious mononucleosis[1].

When to see a doctor [when and where]

I have summarized the urgency in 3 levels. If you are unsure, go by the more urgent one.

Urgency	When	Where
Call 119 right away	Sudden strong abdominal pain (especially upper left; it may spread to the left shoulder, with dizziness, a cold sweat, or a deathly pale face). Difficulty breathing, wheezing, or being unable to breathe unless leaning forward. Abnormal consciousness, a seizure, or a severe headache with a neck that will not bend	Ambulance
Be seen right away, even at night or on a holiday	You cannot swallow saliva or fluids, you cannot drink at all, or you can barely open your mouth (possible peritonsillar abscess)	Emergency department, or a hospital with an ear, nose and throat department
Be seen the same day	Swollen lymph nodes in the neck or strong fatigue have been added to the fever and sore throat. You were told it was tonsillitis but the fever has not gone down and is dragging on. Strong pain on only one side of the throat. A rash appeared after you started an antibiotic. Yellowish skin or whites of the eyes	Fever clinic, ear, nose and throat clinic

* Our clinic is closed on Thursdays. On Sundays and national holidays we run only the fever clinic, 10:00 to 15:00, and cannot give IV drips.

When can I go back to school or work? [there is no fixed number of days for staying home]

Infectious mononucleosis is not an illness for which the Enforcement Regulations of the School Health and Safety Act set a number of days of exclusion from school. Unlike influenza, there is no rule of “so many days after onset”; once the fever is down, you can eat, and you feel recovered, it is fine to go back to school or work. If the school gives instructions about when to return, please follow them.

However, restarting physical education, club activities, and sports is a separate matter. Based on the overseas guides described above, avoid strenuous exercise and contact sports for at least 1 month from onset, and talk with your doctor about restarting.

Even after the fever has gone down, fatigue can remain for a month or more[5]. “I have no fever anymore, but I’m tired” is common with this illness, so ease back into daily life at a pace that is not a strain.

It spreads through saliva, so until you recover, avoid sharing drinks and utensils and avoid kissing. Note, however, that about 15 to 20% of people who have ever been infected shed the virus in their saliva all the time, so it is also an illness that is hard to prevent completely[1].

What to tell us when you see a doctor with a fever and sore throat

If you make a note of the following, it will be easier to tell us at the exam.

How to use our fever clinic [no IV drips on Sundays and national holidays]

A lingering fever and sore throat are eligible for the fever clinic at Sendai Airport North Clinic (Mitazono, Natori City). No reservation is needed; we accept patients through a pre-visit questionnaire and same-day check-in.

Our policy is not to turn away anyone seeking the fever clinic. However, when you cannot drink and need an IV drip, or when we judge that hospital admission or specialized treatment is needed, we may guide you to a medical institution that can provide it.

Day	Hours
Mon, Tue, Wed, Fri, Sat	9:00-12:00 / 14:00-18:00
Sundays and national holidays (from October 4, 2026)	10:00-15:00 (no lunch break, fever clinic only)
Thursday	Closed

The Sunday and national holiday fever clinic runs from 10:00 to 15:00 (no lunch break). We are closed on Sunday, October 25, and from November we are generally open every Sunday and national holiday. On Sundays and national holidays we run only the fever clinic; we do not offer general outpatient care or IV drips.

On Sundays and national holidays we cannot give IV drips. If you cannot even swallow saliva or cannot drink at all, go from the start to a medical institution that can give IV drips.

For details, please see the announcement about the Sunday and national holiday fever clinic and About the fever clinic pre-visit questionnaire and same-day check-in.

What happens during your visit

Frequently asked questions (FAQ)

Can infectious mononucleosis spread to my family or partner?

It spreads mainly through saliva. Please avoid kissing, sharing drinks, and sharing utensils until you recover. However, a 2003 explanation in Japan said that more than 90% of people in their 20s already have antibodies, so the main concern is passing it between young people who have not yet been infected[1]. The incubation period is long, at 4 to 6 weeks, so even if it was passed on, illness would appear well later[1].

When can I restart exercise or club activities?

Because the spleen swells and could rupture, based on overseas guides, avoid strenuous exercise, contact sports, and heavy lifting for at least 1 month from onset. A US study proposed up to 31 days from onset and a UK review proposed an 8-week activity restriction, so policies differ by country and medical institution[9][10]. Please check with the doctor who examined you about when to restart.

I was told it was tonsillitis and started an antibiotic, and then a rash appeared.

It is known that a rash easily appears when amoxicillin and similar drugs are used in infectious mononucleosis[8][1]. Do not stop or continue the medicine on your own judgment; contact the medical institution that prescribed it the same day. Please tell them the day the rash appeared and the name of the medicine, so that they can work out whether it is strep throat or infectious mononucleosis and choose the medicine again.

When can I go to school or work?

Infectious mononucleosis is not an illness for which the Enforcement Regulations of the School Health and Safety Act set a number of days of exclusion from school. You can go to school or work once the fever is down, you can eat, and you feel recovered. However, exercise such as physical education and club activities is a separate matter; based on overseas guides, hold back for at least 1 month from onset and talk with your doctor about restarting.

How long do the fever and fatigue last?

It is an illness in which a fever of 38℃ or higher often lasts 1 to 2 weeks[1]. In the US study of 150 people, most symptoms and test values were almost back to normal within a month, but fatigue and neck swelling improved only slowly[5]. Having lingering fatigue without fever is common with this illness.

Can one blood test give the answer?

The white blood cell breakdown and liver function can be learned from one blood draw, and having 10% or more atypical lymphocytes greatly raises the likelihood[4]. EBV antibodies appear differently depending on the stage of infection, so blood may be drawn twice, in the acute phase and 4 to 6 weeks later, to compare[1].

Summary [for a young person’s lingering fever, sore throat, and swollen neck, confirm with a blood test]

If you feel, “This is long for tonsillitis,” that is an important signal from your body. If you are unsure, please consult us early. Please leave the medical care to us.

That’s all.

Disclaimer

This article is intended to provide general medical information and does not replace an examination, diagnosis, or treatment by a doctor. The content is based on medical knowledge at the time of publication, but the appropriate response differs for each individual. If you are worried about your symptoms, please consult a medical institution.

References

1. Japan Institute for Health Security, Infectious Disease Information site. Infectious mononucleosis (IDWR 2003 Week 23) (in Japanese). Japan Institute for Health Security website. Accessed October 2026. https://id-info.jihs.go.jp/infectious-diseases/infectious-mononucleosis/detail/index.html
2. Putukian M, McGrew CA, Benjamin HJ, Hammell MK, Hwang CE, Ray JW, et al. American Medical Society of Sports Medicine Position Statement: Mononucleosis and Athletic Participation. Clin J Sport Med. 2023 May. doi:10.1097/JSM.0000000000001161 PMID: 37186809
3. Takeuchi K, Tanaka-Taya K, Kazuyama Y, Ito YM, Hashimoto S, Fukayama M, et al. Prevalence of Epstein-Barr virus in Japan: trends and future prediction. Pathol Int. 2006 Mar;56(3):112-6. doi:10.1111/j.1440-1827.2006.01936.x PMID: 16497243
4. Ebell MH, Call M, Shinholser J, Gardner J. Does This Patient Have Infectious Mononucleosis?: The Rational Clinical Examination Systematic Review. JAMA. 2016 Apr;315(14):1502-9. doi:10.1001/jama.2016.2111 PMID: 27115266
5. Rea TD, Russo JE, Katon W, Ashley RL, Buchwald DS. Prospective study of the natural history of infectious mononucleosis caused by Epstein-Barr virus. J Am Board Fam Pract. 2001;14(4):234-42. PMID: 11458965
6. Ministry of Health, Labour and Welfare, Health and Life Sanitation Bureau, Infectious Disease Control Division. Guide to the Appropriate Use of Antimicrobials, 4th edition, Clinical Practice (Outpatient) (section on acute pharyngitis) (in Japanese). Ministry of Health, Labour and Welfare. January 16, 2026. https://www.mhlw.go.jp/content/10900000/001630903.pdf
7. Japan Society of Otorhinolaryngology and Head and Neck Surgery. Representative diseases handled by otorhinolaryngology and head and neck surgery: diseases of the mouth and pharynx (acute tonsillitis and peritonsillar abscess) (in Japanese). Japan Society of Otorhinolaryngology and Head and Neck Surgery website. Accessed October 2026. https://www.jibika.or.jp/modules/disease/index.php?content_id=22
8. Chovel-Sella A, Ben Tov A, Lahav E, Mor O, Rudich H, Paret G, et al. Incidence of rash after amoxicillin treatment in children with infectious mononucleosis. Pediatrics. 2013 May;131(5):e1424-7. doi:10.1542/peds.2012-1575 PMID: 23589810
9. Sylvester JE, Buchanan BK, Paradise SL, Yauger JJ, Beutler AI. Association of Splenic Rupture and Infectious Mononucleosis: A Retrospective Analysis and Review of Return-to-Play Recommendations. Sports Health. 2019;11(6):543-549. doi:10.1177/1941738119873665 PMID: 31550435
10. Bartlett A, Williams R, Hilton M. Splenic rupture in infectious mononucleosis: A systematic review of published case reports. Injury. 2016 Mar;47(3):531-8. doi:10.1016/j.injury.2015.10.071 PMID: 26563483

Related articles on a sore throat and fever

For the fever clinic on Sundays and holidays, see the announcement about the fever clinic on Sundays and holidays; for how to visit, see the page on the fever clinic pre-visit questionnaire and same-day check-in.