A number of diseases are associated with our aging immune systems.

Infectious diseases
Immune senescence can permit the reactivation of old infections, such as Herpes zoster or shingles, which is caused by the chicken pox virus. Immune senescence can also cause the reactivation of latent tuberculosis.

The decline in adaptive immunity can cause infectious agents to be far more potent in older adults than in younger people. This is seen in the high death rates associated with pneumonia and influenza in older adults.

Other physical changes associated with aging contribute to the decline in resistance to infection. These include the decreased cough reflex, which allows bacteria entry into the lungs, and changes in kidney function and, in men, prostate troubles, that can lead to an increased risk of urinary tract infections. Loss of the ability to secrete acid in the stomach in older adults can also predispose to certain infections.

Cancer
There is speculation that the increasing incidence of cancer in aging adults may be related to the decreased ability of the aging immune system to recognize and destroy cancerous tissues.

Autoimmunity
The ability of the immune system to recognize the body as "self" diminishes with age and contributes to an increase in what are called autoimmune diseases, diseases in which we make antibodies to some protein native to our bodies. Such autoantibodies then attack the organs with which those proteins are associated. Some examples of this include diabetes, hypothyroidism, and rheumatoid arthritis. Despite the increased incidence of autoimmune diseases among older adults, most of the classic autoimmune diseases arise most often in our 30's and 40's.