Clinicians often tell patients that the cause of their symptoms is “just a virus.” For example, there are all sorts of viruses that can make a patient suddenly feel fatigued, or simply not well. Maybe there’s a low-grade fever; more commonly, the person feels like they have a fever, but the temperature is normal (that’s always disappointing to me, personally, because then I can’t take off work). Maybe they have cold symptoms, maybe upset stomach and/or diarrhea, but maybe not. Such viruses get better on their own, sometimes in 1-2 days, always within a week.
We never identify which exact virus is responsible, because there are so many possibilities, and clinically, it doesn’t matter. They’ll get better; antibiotics don’t work at all. Of course, there are many specific viruses, some of which can do bad things, such as Herpes, HIV, Covid-19, Rabies, different Hepatitides (plural of Hepatitis), Ebola, and more. This is never what we mean when we say “just a Virus.”
See also Differences Among Germs; also Common Cold, Gastroenteritis (for diarrhea), and our diagnostic approach to a person with Fatigue.