EPIDEMIC | Long-Term Environmental Change

epidemic | the world of infectious diseases

HUMAN CHANGE: MIXED RESULTS

Human populations can evolve new genetic traits, such as resistance to malaria, although this process takes many generations. In some "malaria belt" regions, human red-blood chemistry has changed in ways that disrupt the life cycle of the malaria microbe. People with this altered trait have a better chance of avoiding the disease.

However, people who inherit this trait are subject to sickle-cell anemia, a potentially fatal blood disease that can occur when a person inherits the sickle cell trait from both parents. Even if these people live in regions free of malaria, the trait—and its health risks—persists.

How Plasmodium protozoa infect a human host:

1. The bite of a female Anopholes mosquito injects Plasmodium protozoa into a human host.

2. Plasmodium travel through the bloodstream to the liver.

3. In the liver, Plasmodium multiply asexually.

4. Plasmodium reenter the bloodstream, multiply in red blood cells, and then burst out, infecting new cells and producing malaria symptoms.

5. Plasmodium are ingested by another mosquito, and the life cycle begins again.

6. Inside the mosquito, male and female forms of Plasmodium reproduce sexually, resulting in thousands of genetically varied offspring.