Flexi Says: Illnesses, accidents, and violence are major causes of death in humans before reaching old age. During old age, the immune system becomes less efficient, so older adults are increasingly susceptible to serious illnesses such as cancer, cardiovascular disease, and pneumonia. Aging occurs for at least two reasons. One reason is that cells are programmed to divide a set number of times. After that, they can no longer divide, so they die out. Another reason is that DNA becomes increasingly damaged through time due to mutagens in the environment.