review of Evolution of Infectious Disease by Paul Ewald

Evolution of Infectious Disease by Paul W. Ewald came out in 1994 and is still a highly interesting and maybe revolutionary technical foundation behind evolutionary biology or evolutionary epidemiology.

I wrote about that somewhat in my earlier review of his later book, Plague Time. But somehow I'd forgotten that the same book and author presented both fundamental ideas that:

1. What we call chronic diseases such as heart disease, cancer etc are caused by infectious agents.

2. Infectious microorganisms change through evolution and we can use that to control their effect on us.

The fundamental concept of this book is to point out that microorganisms evolve just like we all do -- and they evolve in ways that increase their opportunity to increase and multiply.

This seems fairly simple, even obvious, but I know I'd never thought about it before. I've always assumed that a cold germ was a cold germ was a cold germ. It never occurred me to that germs do change over time -- and since many microorganisms multiply so quickly, the changes are easily observable by people, over years if not months.

He takes a lot of time to discover the concept in biology that parasites and hosts always co-evolve to form a beneficial relationship rather than a hostile one.

He points out that this is true only when it's in the best evolutionary interests of the parasite to do so.

In cases where rapidly killing the host will help the parasite spread faster -- guess what, it kills the host quickly.

Thus, the rhinovirus that causes the common cold is relatively benign (that is, non-life threatening even though irritating) virus to people. That is because is spreads through the air by coughs and sneezes and via hands holding objects.

Thus, the rhinovirus can spread to more people if its infected victims feels well enough to go to work, shopping etc -- where they cough and sneeze and spread the cold germs to the rest of us.

If a cold made us feel so sick to death that we had to stay in bed, we would come into contact with only our close families and therefore the virus is rewarded for not killing us, by spreading to more people.

Some viruses spread more easily by making people so sick they stay in bed -- such as malaria. That's because it's spread by mosquitoes, not in person contact. A sick person lying in bed is even more at risk of being bitten by mosquitoes, who then carry the virus to more victims.

Where does AIDS fit in?

HIV is a virus that spreads primarily through sexual contact. Therefore, it is to HIV's advantage to lie low and not cause great illness for many years, to increase the victim's chances of spreading HIV to their sexual partners.

If HIV immediately caused grave illness and opportunistic infections, it could not spread much -- because people with full-blown AIDS are too sick to have much sex and even if they still want to, they're too obviously sick to attract many sexual partners.

Ewald argues convincingly that HIV could be strongly encouraged to evolve to a more benign virus by minimizing the number of sexual partners people have. He backs this up with statistics from Africa, where the regions with social disruption and where people have many sexual partners have a high percentage of the virulent HIV-1. Regions of Africa with social stability and where people are discouraged from sexual promiscuity tend to have HIV-2, which is much less dangerous.

The logic here is that, the more sexual partners an HIV-infected person has, the greater the chance HIV has of spreading to someone new in a relatively short time. Therefore, HIV is better off spreading more quickly and killing its host fasters, when they're have many sexual partners.

How might this apply today to bird flu? I'll discus that in a separate entry.