My sister has mononucleosis. I drank out of her drink before we found out that she had it. Does this mean that I have mono now?
– Kyle*

Mono, or infectious mononucleosis, is caused by an infection with the Epstein-Barr virus (EBV). EBV spreads through direct contact with saliva (spit). This can happen by sharing eating utensils, drinks, and even things like lip gloss, lipstick, or lip balm.

Because it takes about 1–2 months for symptoms to start, people who are infected can spread the virus without knowing it. They're most contagious from right before symptoms start until they go away. But they can stay contagious for months after their symptoms have cleared up. Then, the virus stays dormant (inactive) in the body for the rest of their life. That means it can sometimes show up in their spit and get passed on to someone else. So people can spread EBV even when they're feeling OK.

If you've shared drinks with or kissed someone who has mono, there's no way to tell whether you will get it — unless you know you've had mono before. People who have already been infected with EBV in the past probably won't get sick from EBV again because they have developed antibodies and are immune to it.

Most people have been infected with EBV by the time they reach adulthood. Up to half of kids are infected before age 5. So you may very well be carrying EBV and not know it.

If you do get mono symptoms — such as fever, sore throat, and tiredness — call your doctor to find out whether you need an appointment. If so, your doctor will examine you and may also do a blood test for mono. Other viruses and some bacterial infections — such as strep — may cause similar symptoms.

*Names have been changed to protect user privacy.