The most common and most likely answer to this crossword clue is the 9 letter word PATHOGENS
I've seen this crossword clue in The Guardian
P
A
T
H
O
G
E
N
S
Buddy explains!
The straight part
We know that "viruses?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case viruses? = pathogens.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PATHOGENS mean?
PATHOGENS noun- Any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa or fungi. Microorganisms are not considered to be pathogenic until they have reached a population size that is large enough to cause disease.