As pointed out previously, the immune system is a complicated and delicately balanced system that performs a vital function within the body. If this balance is disrupted, serious trouble can develop.
A suppressed immune system leaves the body wide open to invasion by foreign organisms and cancer cells. This immunosuppression can occur secondarily to viral infections (such as parvovirus or distemper virus in dogs, and the feline leukemia and feline AIDs viruses in cats), drug therapy (including steroid treatments), severe stress, and disorders of the bone marrow.
In addition, pets can actually inherit poorly functioning immune systems.