Final answer:
Parasites carried by insect vectors or water are generally less virulent than those spread by direct contact because their means of transmission does not require the host to be healthy and mobile, thereby not limiting their spread even if the host is severely ill.
Step-by-step explanation:
Parasites transmitted by insect vectors or water tend to be less virulent than parasites that are transmitted by direct contact. This is primarily because parasites with transmission methods that do not depend on close physical contact between hosts (e.g., those transmitted by vectors such as mosquitoes or through water) can afford to be less virulent, as their hosts being debilitated does not significantly reduce the probability of transmission. By contrast, directly transmitted parasites often rely on close interactions between hosts, so a severely debilitated host may reduce the likelihood of the parasite spreading. Therefore, it is selectively advantageous for these parasites to maintain a balance, allowing the host to remain healthy enough to facilitate transmission.
One example is malaria, which is transmitted by mosquitoes that act as vectors. Malaria parasites need to balance their virulence so that they do not incapacitate their human hosts before being able to transfer back to mosquitoes or to other humans. The survival and transmission dynamics of a parasite are intimately linked to their virulence levels, which are influenced by how the parasite moves between hosts.