Expert: Florence M Rollwagen - 7/19/2008

Question
Hello Dr. Rollwagen,

I am a high school student here in Australia and had a question about viruses in populations that I was wondering about. Recently a previously unknown tribe was discovered in South America, and one of the reasons given for the decision to not make further contact with them was the risk of introducing harmful viruses to their population.

Throughout history, when one group of people e.g. explorers, immigrants, conqerors etc. meet a native population e.g. American Indians, Incas, Polynesians etc. there is a repeated theme of the new population bringing viruses which the native population have no adequate immune response for, e.g. introduced smallpox into American Indian tribes.

My question is, why doesn't such infections work the other way around? Surely the native population would have also evolved particular strains of viruses which would be unknown to the immune responses of the visiting population?

Any help you could give would be much appreciated.
-Many Thanks,
Rohan.

Answer
Hi Rohan:  Thanks for your question.

An excellent question, by the way, and one that shows that you have been thinking about things.

The possibility of harmful viruses being passed both ways when populations meet is certainly a concern.  But I think that we're playing a numbers game here.  Let's say that our isolated population (Q) has about 3,000 inhabitants (I made that up).  A virus that killed 30% of the population (like the Black Death in Europe) would kill about 900 of our isolated population.

If the virus went the other way, and killed 30% of the outside population (how many billions are we?), do you think that would have a noticeable effect?  Probably not.

So the risk is greater for the smaller population, since a large loss of genetic material could result in a genetic bottleneck (look it up) and eventual loss of the population.

Hope this helps.  Please write back if you have more questions.

FM Rollwagen, PhD