Counters a disease vector, cutting off half (5)
I believe the answer is:
abaci
'counters' is the definition.
(plural of abacus, something used for counting)
'a disease vector cutting off half' is the wordplay.
'disease vector' becomes 'bacillus' (bacillus is a type of disease-causing bacterium).
'cutting off half' means to take half the letters.
'bacillus' halved is 'baci'.
'a'+'baci'='ABACI'
(Other definitions for abaci that I've seen before include "sliding scales for calculation?" , "Counting devices" , "summers" , "frames" , "These help with numbers" .)