Anti-
Prefix generally meaning 'against, opposite or opposing, and contrary.' In medicine, anti- often connotes 'counteracting or effective against' as in antibacterial, anti-infective, and antiviral.
Sometimes medical terms containing anti- take on new meanings as has occurred with antibiotic and antibody.
As a prefix, anti- may be shortened to ant- as in antacid.
'Anti' is the Greek word for 'against.