With time, the immune system becomes less efficient. This process begins when the thymus starts to atrophy, which begins right after puberty. The immune system’s decline therefore starts very early on, but its first symptoms are rarely discernible before we reach our fifties. The size of the thymus, as well as its activity, progressively withers with age. The organ’s weight diminishes by two thirds and its lymphocyte content by 90%. The levels of thymic hormones in the blood also decrease after puberty and reach their lowest levels starting at around sixty years old. This increasing scarcity explains the deterioration of immune system performance observed in the elderly. Indeed, aging reduces our resistance to tumors and is often accompanied by more frequent infections and higher related mortality. In older people with poor immune response, all-cause mortality is twice as high as in young subjects. Cancer mortality is for its part three times as high, as are lung infections, whose propagation is helped by the decrease of the cough reflex. Latent infections like shingles, herpes or measles can resurface, but the most frequent risk is that of repeated urinary infections which gradually become resistant to antibiotics.