A virus is the smallest parasite that exists, usually ranging from 0.02 to 0.3 μm in size, according to people who know what an μm is.
I never came across it. I think I chanted it once. I expect that viruses must feel short-changed, given that they don’t even reach a whole μm. Which must make them really self-conscious ... when they meet up with creatures strutting about at a hefty one or two μms.
Anyway, a virus or virion (if you want to sound Elizabethan) contains a simple nucleic acid (RNA or DNA) surrounded by enzymes that prompt replication – like a viral Viagra. But they can only replicate when they get inside the cells of animals and plants, causing all manner of infectious symptoms, especially among humans, who have now had to invent elbow-bumping. (Latest greeting out of China: foot-bumping. True.)