Host and Pathogen Evolution

is studying these and other diseases in terms of genetics is that the human genome is large and it is laborious to look at the entire genomes of many people in order to find the necessary trends.

Still, there are some diseases that have been linked to certain genes in humans. An example of this is the presence of sickle cell gene and malaria resistance. People who have the Duffy blood group or Melanesian ovalocytosis also have protection against malaria. People with a prion protein gene variant are protected from Creutzfeldt-Jakob disease and those with a CCchemokine receptor 5 mutation do not get HIV-1 disease to a great degree. Being a blood group non-secretor protects against Norwalk viral infections.

There is some evidence that there are genes affecting the innate immune system that confer a decreased risk to a number of infectious diseases, particularly if the person is heterozygous for the genes. Most of the diseases that have been studied are chronic diseases like malaria, tuberculosis, and leprosy. Related to this is the finding of certain genes that increase the risk of autoimmune diseases, particularly in European populations.

HOST AND PATHOGEN EVOLUTION

A pathogen thrives if it can survive, reproduce, and spread to a different host. Parasites, host immunity, predators, and limitations in the environment all exert evolutionary pressures on the survivability of a particular pathogen. We have already talked about the evolutionary arms race, in which the pathogen and host continually adapt in order to outdo each other. The hosts that have resistance are selected for and the pathogen that can successfully beat those resistances are also selected for.

There are physical barriers that first protect the host from disease. Pathogens have evolved to have toxins and enzymes that can break through these defenses. Host complement systems also protect the host but there are enzymes in the bacterial species that can get through these defenses as well. Other defense mechanisms include sequestering host resources, such as iron, which is sequestered in the liver, lymph cells in the immune system, and antibiotics made by the host that can fight off disease.

The previous chapter on coevolution explained how it is coevolution that causes the changes in the host and pathogen over time so that both the host and pathogen can survive. Because it takes much longer for human and higher animal evolution to occur compared to that of bacterial and viral organisms, mankind has augmented the lack of evolution through the invention of