This could be a sign of a ruptured spleen, a rare but serious complication of mononucleosis.
What Is Mono?
Mononucleosisis a very common infectious disease usually brought on by theEpstein-Barr virus(EBV).

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For most people, mono isn’t very serious and will improve over time with proper treatments.
Generally, mono goes away after two to four weeks.
For some, the fatigue may last longer, up to six months after getting sick.
Is Mono Herpes?
EBV, the virus that causes most cases of mono, is a kind of herpes virus.
Is Mono Contagious?
Mono is very contagious.
When it spreads through sexual contact, mono could be considered an STD.
However, that is not the only way it spreads.
How Does Mono Spread?
A common way of transmitting mono is by sharing drinks or eating utensils.
Condomless sex and kissing are also easy ways to spread mono.
Who Is at Risk?
Younger children and older adults can also get mono, but their symptoms tend to be milder.
Can You Get Mono More Than Once?
One you have had mono, the virus remains in your body but becomes inactive.
Although some research suggests that it may be possible for mono to recur, this is considered very unlikely.
Treatment
There is no vaccine for mono, and mono is not curable.
It will, however, go away on its own.
Treatment revolves around doing what you’re able to to alleviate and manage your symptoms.
These include:
You should also avoid contact sports that could potentially cause your spleen to rupture.
Healthcare providers may also recommend that you avoidpenicillinantibiotics, such as amoxicillin, if you have mononucleosis.
Getting a diagnosis can help you avoid spreading the virus to other people.
Todiagnose mono, your medical provider will assess your symptoms and check for swollen lymph nodes in your neck.
They can usually determine when someone has mono by symptoms alone.
You may need blood tests if you don’t have a typical case of mono.
Mononucleosis is extremely contagious.
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