Final answer:
A healthy human being has a strong immune system that can defend against pathogens. However, as people age, their immune system becomes less effective, making them more susceptible to diseases.
Step-by-step explanation:
The average human being with a healthy immune system is capable of putting up a strong defense against pathogens. However, as people age, their immune system naturally becomes less effective, which is known as immunosenescence. This age-related decline typically begins around the age of 50 and worsens with increasing age, making older individuals more susceptible to diseases.