The most common and most likely answer to this crossword clue is the 5 letter word COLDS
I've seen this crossword clue in Independent
C
O
L
D
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 = colds.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 COLDS mean?
COLDS noun- A condition of low temperature.
- (with 'the') A harsh place; a place of abandonment.
- A common, usually harmless, viral illness, usually with congestion of the nasal passages and sometimes fever.
- Rheum, sleepy dust