An opportunistic infection is an infection caused by pathogens, particularly opportunistic pathogens – those that take advantage of certain situations – such as bacterial, viral, fungal, or protozoan infections that usually do not cause disease in a healthy host, one with a healthy immune system. A compromised immune system, however, presents an “opportunity” for the pathogen to infect.

Treatment depends upon the type of opportunistic infection, but usually involves different antibiotics. Opportunistic infections caused by Feline leukemia virus andFeline immunodeficiency virus retroviral infections can be treated with lymphocyte T-cell immune modulator.