What age-related change increases the risk of infections such as influenza?

The correct answer highlights that as people age, there is a notable decrease in the efficacy of the immune system, leading to an increased susceptibility to infections, including influenza. This phenomenon is part of immunosenescence, which is characterized by a decline in both the adaptive and innate immune responses.

Older adults may experience a reduced number of naive T-cells due to decreased thymic output and alterations in B-cell function, which impede the body's ability to mount effective responses to new infections. The overall reduction in immune function not only allows for a higher risk of infections but can also lead to more severe manifestations of these diseases.

In this context, the other options do not appropriately address the key issue related to age-related changes in immunity. For instance, while decreased skin turgor relates to skin integrity and the prevention of skin infections, it does not directly correlate with the immune response to respiratory pathogens like influenza. Increased CD4 T-cells and increased autoimmunity are generally not features associated with aging; instead, older adults often exhibit decreased CD4 T-cell function and a tendency for autoimmune diseases may decline in certain contexts due to a reduced immune response. Therefore, the increased susceptibility to infections is the most accurate and relevant choice regarding age-related changes affecting the