Final answer:
The frequency of accidents and susceptibility to colds and allergies decline during early adulthood due to peak physical health and immune system development. The hygiene hypothesis suggests that exposure to pathogens in childhood strengthens the immune system, leading to fewer allergies and colds in adulthood. Late adulthood sees an increase in susceptibility due to immunosenescence.
Step-by-step explanation:
The frequency of accidents and susceptibility to colds and allergies declines during early adulthood, a period characterized by peak physical health. We see a decrease in such incidents partly because of the development of the immune system that continues from childhood into early adulthood. Moreover, the immune system's exposure to pathogens during childhood, as suggested by the hygiene hypothesis, may lead to a stronger immune response to pathogens and allergens later in life.
According to the hygiene hypothesis, individuals in industrialized countries are exposed to fewer pathogens early in life due to higher standards of cleanliness and smaller family sizes. This underexposure can lead to a higher incidence of allergies and weaker immune responses as these individuals reach adulthood. In contrast, regular exposure to pathogens during childhood in less industrialized countries may contribute to a more robust immune system, with fewer allergies and a lower susceptibility to colds as adults.
Therefore, although the immune system weakens with age (immunosenescence), early adulthood is typically when people experience a decline in accidents and are less susceptible to colds and allergies. This condition changes during late adulthood, with individuals generally becoming more susceptible to diseases due to a natural decline in the immune system's effectiveness starting at about age 50.