vir(o)- Also viro- and viru-.
[Latin virus, poison.]
Viraemia (also viremia) is the presence of viruses in the blood; a virucide or viricide is a substance that kills viruses; a virion is the complete, infective form of a virus outside a host cell; virology is the branch of science that studies viruses.
English virulent shares the same Latin root. However, virile and virility come instead from Latin vir, man, while viridian and viridescent derive from Latin viridis, green.