In some circumstances, precautions can be taken when it is known that defenses are compromised—for example, during surgical procedures and when people are likely to be exposed to particularly dangerous pathogens. The choice of precautions depends on the potential route of exposure and infection, and it can range from wearing a mask in an area where airborne pathogens could be a high threat; to wearing gloves and protective clothing to avoid direct transmission of pathogens; to isolating patients in clean rooms where air is filtered, food and water sterilized, surfaces disinfected, and anyone who enters is masked and gowned to avoid exposure or transmission.
Many biological agents considered to be the most serious disease threats enter the body via the respiratory system. In some cases, only direct environmental exposure would cause disease (there is no person-to-person transmission). For example, B. anthracis spores can be inhaled, ingested with contaminated food or water, or contacted by the skin on contaminated surfaces. In many cases, infected people pose no risk to others. In other cases—such as smallpox and plague—the risk of inhalation from intentionally contaminated environments is complicated with the risk of interpersonal transmission. Successful introduction of an agent via the airway is possible under specific conditions. The optimal size for a particle that contains the infectious microorganism is about 5 micrometers (µm) for efficient penetration to the lower airway (within the lung). However, smaller or larger (up to 12 µm) particles can cause disease by entering the upper respiratory tract (Davis, CUBRC, Inc., 2004 personal communication).
To cause disease, in most cases, the pathogenic agent must enter and multiply in the cells of the host’s body. In the laboratory, each intact and complete bacterial or viral particle has the ability to multiply. However, virulence and infectivity can vary within a bacterial or viral population and among different strains of the same species. In the host, successful infection, multiplication, and resulting disease can be a rare event. Normally, host defenses are overcome only when challenged simultaneously by many of the same type of pathogenic microorganism; the number needed to cause disease is called the infectious dose. If the agent begins to multiply in an infected cell, the cell will be altered to become a potential target of the immune system. The likelihood that an infection will lead to disease depends on how many microorganisms infect the host initially, the nature of the agent (some are naturally better able to overcome the host’s defenses), and the state of the host’s immune system, which varies within a population. For example, young children and elderly people often have weaker immune systems than healthy adults do. In addition, there is an increasing number of people who are more susceptible than average in the U.S. population because they are immunocompromised, either due to a genetic deficiency, because of a metabolic disease, or as a side effect of therapy for an illness such as cancer.