The most common and most likely answer to this crossword clue is the 3 plus 5 letter word BAD APPLE
B
A
D
A
P
P
L
E
More clues leading to the result BAD APPLE
Buddy explains!
The straight part
We know that "it'll infect the others" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it'll infect the others = bad apple.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.