Final answer:
The average human being with a healthy immune system is usually able to put up a strong defense against pathogens. However, as people age, their immune system naturally becomes less effective, leading to a decline in immune function called immunosenescence.
Step-by-step explanation:
The average human being with a healthy immune system is usually able to put up a strong defense against pathogens, which are tiny disease-causing organisms. However, as people age, their immune system naturally becomes less effective, leading to a decline in immune function called immunosenescence. This age-related decline generally starts around age 50 and worsens with increasing age, making older people more susceptible to diseases.