Many diseases, drugs, and other treatments can cause the body’s natural defenses to break down. Such a breakdown can lead to infections, even by microorganisms that normally live harmlessly on or in the body. For example, the risk of infection is greatly increased in animals with extensive burns because damaged skin cannot prevent invasion by harmful microorganisms. Drugs such as the anticancer drugs used in chemotherapy can suppress the immune system. Radiation treatments can also suppress the immune system.
The ability to fight certain infections decreases dramatically in animals with immune disorders. Such animals are at particular risk of opportunistic infections (infections by microorganisms that generally do not cause infection in animals with a healthy immune system). They also become more severely ill from many common infections. Older animals are also more susceptible to infections because aging reduces the immune system’s effectiveness.