doubled over the next 5 million years. Scientists believe that it remained relatively stable into the 18th century. In the 250 years between then and now, however, life expectancy more than doubled again due to improvements in sanitation and healthcare.
We live in a time of high average life expectancies. However, our long evolutionary history has adapted us for different lifestyles (and even life expectancies), and these have changed drastically.As a result, immunity not only weakens in older age; it also becomes imbalanced.
This affects the two branches of the immune system — “innate” immunity and “adaptive” immunity — in a double whammy of “immunosenescence.”“Innate” immunity, which is our first line of defense against infections, fails.