DAMAGE AND REPAIR
In the contest with their host, pathogens must rob the host to secure their own nourishment. Various bacteria and the proto- zoa that causes amoebic dysentery secrete enzymes that digest nearby host tissues and then absorb the products of digestion.
Others literally eat through host tissues, for example, filaria worms, which live in the anterior part of the eye, or the larvae of another species of worm, Angiostrongylus cantonensis, which burrow through the brain. Both of these defend themselves with secretions that inhibit inflammation. Still others, such as the trypanosomes, a group of protozoans that cause diseases such as African sleeping sickness, live in the bloodstream and absorb nutrients directly from the plasma. Whatever the means, parasites secure their resources from the host and then use them for their own maintenance, growth, and reproduction.
These activities of pathogens incidentally damage the host, but this damage is not a pathogen adaptation. It does not do a tapeworm any good to have its host malnourished. It does not do the malarial parasite any good to destroy its host’s blood cells (unless, perhaps, this frees up iron for use by the parasite). Most often, the opposite must be true. The survival and well-being of the parasite depend on the host’s continued survival and ability to provide it with nourish- ment and shelter. Such incidental damage must therefore be consid- ered a cost to both host and pathogen.
