Parasitism is an important source of mortality - in many species, including humans.
However, it appears to have been widely neglected by theorists as a possible contributior
Here, three theories of parasite-driven senescence are presented:
Parasite adaptation to hosts over host lifetimes is part of a more general theory of senescence, that also includes native cells, and helps to explain phenomena such as cancer. This is sometimes called endogenous parasitism - and is covered in this essay's references.
- Accumulation of persistent infections - Many pathogens persistently infect hosts. These accumulate over the lifespan, causing more and more damage until the host dies.
- Parasites adapt to their hosts - Persistent parasites get better at adapting to their hosts over the lifetime of their host, with within-organism selection favouring those that damage their host the most.
- Parasites deplete finite defensive resources - In some cases, hosts have finite defensive resources, which are gradually destroyed by parasites. Tooth enamel is an example of such a resource.
Let's hope that parasite-driven senescence is studied in more detail in the future.