viruso

Viruso is one of the words that quietly dates the Esperanto vocabulary in a good way. The root came in from Latin with medicine, and then in the 1980s the whole world extended it to software at the same time, so Esperanto extended it too without needing a committee. Both senses are current, standard, and distinguished only by context, exactly as in English.

The medical sense is the primary one. Virusa malsano is a viral illness, viruso de la gripo is the flu virus, and kontraŭvirusa covers both the antiviral drug and the antivirus program, which is either a happy accident or a small joke depending on your mood. Note that a virus is not a bacterium: bakterio is the other thing, and medicines that work on one do not work on the other, which is worth knowing in any language.

Keep it away from kancero, which sits a dozen ranks along in this same chapter and names something entirely different: cancer is a disease of the body's own cells, not an infection by an outside agent. The two words end up in the same conversations and mean nothing like each other. And do not confuse the root with viro, a man, which is a different word with a different vowel doing different work.