Oncogens are genes which are involved in the regulation of cell growth. If these genes are defect or do not express correctly, it can lead to uncontrolled cell division and, in extreme cases, to the formation of tumours.

Opportunistic germs: Pathogens which cause illnesses only in patients with weakened immune defences, such as is the case with AIDS, are called opportunistic germs.

Opsonization: The change in the surface of a pathogen or another foreign body so that they can be ingested by pathogens is called opsonization. Antibodies and the complement system opsonize extracellular bacteria, preparing them for destruction by neutrophile cells and macrophages.