Disease Transmission

In talking about epidemiology, it is important to review how outbreaks occur. First, we’ll look at three commonly used and often misunderstood terms: “epidemic,” “outbreak,” and “cluster.” An epidemic is the occurrence of more cases of disease than would normally be expected in a specific place or group of people over a given period of time. To an epidemiologist, “outbreak” means basically the same thing. In the public’s mind, however, “epidemic” has a far more serious connotation than “outbreak.” For this reason, “outbreak” is often used to avoid sensationalism. The third term “cluster” is occasionally used incorrectly in place of “epidemic” or “outbreak.” A cluster is a group of cases in a specific time and place that may or may not be greater than the expected rate. Often the aim of investigating clusters is to determine the baseline rate of disease for that time and place. Two other terms you will come across are “endemic” meaning a high background rate of disease, and “pandemic” meaning very widespread often global disease.

For an outbreak or epidemic to occur, the basic elements of disease causation and an adequate chain of transmission must be present. Disease occurs when an outside agent capable of causing the disease meets a host that is vulnerable to the agent in an environment that allows the agent and host to interact. Then, given a chain of transmission from one host to another and a suitable mode of spread, an outbreak can develop. These basic concepts help guide the selection of public health strategies to prevent health problems. Depending on which approach might be most effective, we might direct efforts at the specific agent (e.g., guinea worm), host (e.g., immunization to prevent measles), or environment (e.g., sanitation improvements to prevent salmonella). We can also target a specific point in the chain of transmission. This was the response in the E.coli outbreak in Washington State in the early 1990s when health officials called for the thorough cooking of hamburgers to interrupt transmission of the bacterium.

The host is the person, or in a more generic definition the organism, that is susceptible to the effect of the agent. The status of the host is quite important and is generally classifiable as susceptible, immune, or infected. Finally, and also quite important, is that the host’s response to exposure can vary widely from showing no effect to manifesting illness.

The environment is the conditions or influences that are not part of either the agent or the host but that influence its interaction. A wide variety of factors, including physical, climatologic, biologic, social, and economic conditions, can come into play. For instance, in a study of motor vehicle injuries, the agent (mechanical energy) and the host (driver) could be affected by the topography, the weather, and the actions of other drivers. In many infectious disease outbreaks, social and economic conditions cause overcrowding and lead to high levels of exposure.

Agent, host and environment alone are not sufficient to cause an epidemic; an adequate chain of transmission must be present. This process requires a source for the agent, a portal of exit, a mode of transmission, and a portal of entry. The first element, the source for the agent, is often the place where the agent originates, where it lives, grows, and multiplies, but this is not always the case. The agent that causes botulism (Clostridium botulinum), for example, originates in soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores.

The second element, a portal of exit, is a pathway by which the agent can leave the source. This pathway is usually related to the place where the agent is localized. For instance, the agents causing tuberculosis and the flu are released through the respiratory tract, whereas agents for many stomach ailments are released through the digestive tract. Agents found in the blood, such as hepatitis B and HIV can be released through cuts or needles.
Once the agent leaves the source a mode of transmission, or means of carrying it to the host, is needed. This can happen in a number of ways some of which are direct and some indirect. Direct transmission includes contact with soil or plants as well as contact between people. In indirect transmission, the agent can be airborne, vector borne, or vehicle borne. In airborne transmission, the agent is carried from the source to the host suspended in air particles. Vector-borne diseases are transmitted indirectly by a live carrier usually an arthropod such as mosquitos, fleas, or ticks. Vehicle-borne diseases are carried by inanimate objects such as food or water, blood, or items like handkerchiefs, bedding, and surgical instruments.

Finally, there must be a pathway into the host, a portal of entry, that gives the agent access to tissue where it can multiply or act. Often the agent enters the host in the same way that it left the source. This is the case with the flu virus which leaves the source through the respiratory tract and enters a new host through the respiratory tract.