Which prefix indicates opposition or being against?

The prefix "contra-" specifically indicates opposition or being against something. It originates from Latin, where "contra" means "against." This prefix is often used in various medical terms to denote a condition or a factor that counteracts or opposes. For example, the term "contraceptive" refers to methods that prevent pregnancy, literally meaning 'against conception.'

The other prefixes do not convey the same meaning of opposition. "Poly-" means many or much, referring to a number of something, "pseud/o-" indicates false or deceptive, often used in terms that describe something that is not genuine, and "bio-" relates to life or living organisms. Therefore, "contra-" is the only prefix that fits the description of indicating opposition or being against.