Summary of Chapter Concepts

Many different types of parasites affect the abundance of host species. Ectoparasites live on host organisms whereas endoparasites live in host organisms. As a group, parasites include a wide range of species that include plants, fungi, protozoa, helminths, bacteria, viruses, and prions. Among parasites that cause diseases—known as pathogens—those that have recently become abundant are called emerging infectious diseases.

Parasite and host dynamics are determined by the parasite’s ability to infect the host. The transmission of parasites can be horizontal—either through direct transmission or transmission by a vector—or vertical from parent to offspring. The ability to infect a host also depends on the parasite’s mode of entering the host, its ability to infect reservoir species, its ability to jump to new host species, and its ability to avoid the host’s immune system.

Parasite and host populations commonly fluctuate in regular cycles. These fluctuations occur because transmission increases with host density but decreases as an increased proportion of the host population develops immunity. These fluctuations can be modeled using the S-I-R model.

Parasites have evolved offensive strategies while hosts have evolved defensive strategies. Natural selection has favored parasites that can improve their probability of transmission, including manipulations of host behavior. Hosts have evolved both specific and general immune responses to combat host infection. Hosts also can employ mechanical and biochemical defenses against parasites. Coevolution occurs when the parasite and host continually evolve in response to each other.