The transmission of infection involves several stages, during which the infectious agent is present in the host, and other times when it is in the external environment, a vector, or both. Both internal and external environments present hazards to infectious agents.

The host has its own natural defense mechanisms, including surface-active chemicals, specific reactive cells, phagocytes, and humoral antibodies. The successful parasite must be able to avoid, in part, these mechanisms and must also avoid competition with other agents that may simultaneously infect the host in a similar niche. Parasites have evolved strategies to resist the host’s protective mechanisms, such as acid-resistant helminth cuticles (to resist gastric acid), an intracellular mode of life, encystment, immunosuppression, and reduced immunogenicity. Some bacteria possess capsules that protect them against phagocytosis. Many parasitic nematodes have a greater fecundity than their free-living counterparts, thus ensuring that some offspring will survive the host’s immune response and potentially lethal conditions in the external environment.

The two main hazards presented by the external environment are desiccation and ultraviolet light. Desiccation is not always lethal, but it frequently inhibits multiplication. Low temperatures are not usually lethal, but can inhibit multiplication. The high temperatures attained in temperate climates are probably not lethal, but those reached in tropical countries may be more effective. Many agents may be partially protected from desiccation by being discharged in moist carriers such as feces and urine. They also may persist by being shed into favorable surroundings.

Maintenance strategies: The ways in which infectious agents are maintained can be considered as strategies for maintenance. Five main strategies can be identified:

• Avoidance of a stage in the external environment
• The development of resistant forms
• A ‘rapidly in, rapidly out’ strategy
• Persistence within the host
• Extension of host range

Avoidance of a stage in the external environment

Some agents avoid transfer via the environment. There are four main methods: vertical transmission, venereal transmission, vector transmission, and transmission by sarcophagia (flesh eating). For example, the helminth Trichinella spiralis occurs in cysts in the muscle of pigs, rats, and other animals, and is only transmitted when these animals are eaten by predators and scavengers, including man.

Resistant forms: The harshness of the external environment can be buffered by surrounding the infectious agent with a shell that is resistant to heat and desiccation. Some bacteria form such shells (spores). Examples include members of the genera Clostridium and Bacillus, which can survive boiling water, even flames, for short periods of time, and may survive in the external environment for decades. Fungi also may produce spores. Generally, these are less resistant than bacterial spores. Some helminths and protozoa form resistant shells (cysts). These can protect the agent from the host’s defense mechanisms; the protozoan parasite, Toxoplasma gondii, for example, can survive for many years in its cystic form in the host, until the latter is eaten. Thick-shelled helminth eggs can resist the external environment and may overwinter on pasture.

Rapidly in, rapidly out’ strategy: Some agents enter the host, replicate, and leave very quickly, before the host has time to mount an immune response or die. Many viruses of the upper respiratory tract can do this within 24 hours. The strategy requires a continuous supply of susceptible hosts. This may be one reason why respiratory and enteric infections, such as the common cold virus in humans, are not present in primitive societies of low population density, and may not have occurred in small prehistoric societies.

Persistence within the host: Infectious agents may persist within the host, sometimes for life. Persistence occurs because the host’s defense mechanisms fail to eliminate agents. This failure may arise because microorganisms adapt to the host’s phagocytic cells or develop strategies for avoiding the host’s immune response. The latter includes immunosuppression and tolerance. Immunosuppression results in the agents being maintained in the host for varying periods of time. It may be general or antigen-specific. General immunosuppression is demonstrated by some viruses (e.g., rinderpest) and protozoa (e.g., Toxoplasma gondii), and facilitates survival of these and other agents in the host. Tolerance is due to a primary lack of responsiveness by the host, rather than active suppression. Tolerance is sometimes found in infections with microorganisms that have antigens like normal host antigens. Other means of avoiding the host’s immune response are antigenic variation, intracellular parasitism, multiplication in sites inaccessible to the immune response, and the induction of ineffective antibodies.

Extension of host range: Many infectious agents can infect more than one host. Extension of the host range is an obvious way of maintaining infection, and is facilitated by the presence of various hosts in the same area. However, if an agent is present in different species in the same region, it should not be assumed that transfer between them always occurs.