Mononucleosis
“Mono can mimic the flu, although there’s no seasonality to it,” says Dr. Cloonan. Caused by the Epstein-Barr virus, the illness usually brings on a fever, fatigue, sore throat, and swollen lymph nodes in the neck and other parts of the body; nearly nine out of 10 people will get sick with this virus at some point in their life. “The younger you are when you’re exposed to it, the less sick you tend to get,” he says. “You may not know a young child has it because kids are sick so often and the illness may not be severe.”