Immunodeficiency diseases manifest clinically as a predisposition to infections. They are usually recognized when an animal makes multiple visits to a veterinarian for infections that would normally be relatively easy to control. Two major groups of immunodeficiency disease occur. One group is inherited as a result of mutations or other genetic disease. These primary or congenital immunodeficiency diseases usually develop in very young animals (<6 mo old). The second group of immunodeficiency diseases are secondary to some other stimulus such as a viral infection or tumor. These secondary or adaptive diseases tend to occur in adult animals. One other general rule in diagnosing immunodeficiencies is that defects in the innate and antibody-mediated immune systems tend to result in uncontrollable bacterial infections, whereas defects in the cell-mediated immune system tend to result in overwhelming viral and fungal infections.