Final answer:
Pathogens often become less lethal over time due to natural selection, which favors less virulent strains as they allow longer transmission periods. Host immunity also plays a key role, with genetics, health conditions, and environmental factors influencing susceptibility to infections.
Step-by-step explanation:
Over time, pathogens often become less deadly due to natural selection, which favors strains that are not too lethal to their hosts. A dead host can't transmit the pathogen to new hosts, so those strains of the pathogen that allow for longer periods of transmission tend to spread more successfully. Additionally, over the course of multiple infections, host populations can develop immunity or resistance to certain pathogens, which also drives the selection of less deadly strains.
A pathogen's virulence, or its capacity to cause damage, is largely determined by its reproductive success within the host and its ability to evade or overcome the host's immune defense mechanisms. For the host, a combination of several factors, including genetic predisposition, preexisting health conditions, and environmental factors like sanitation, affect the severity of the infection and the risk of transmission to others.
Thus, the cause-and-effect relationship between the pathogen's virulence and the actions taken by the host, whether those be immune responses or medical interventions, leads to a natural attenuation of the pathogen over time. The use of antibiotics and the practice of vaccination are medical strategies that have a significant impact on reducing both the prevalence and the virulence of pathogens.