Mrs. wilson is 78 years old and over the past few years she has become increasingly susceptible to bouts of pneumonia. this can most clearly be attributed

A weak immune system

Explanation:

When people get older their immune system becomes weak including the function of the T cells which are cells responsible for fighting infections.

This means it becomes hard to recover from a flue when someone get sick and they are always prone to sickness since their immune system no longer has the power to fight infections.

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