Fig. 9.6 Routes of entry and dissemination of microbes. To enter the body, microbes penetrate epithelial or mucosal barriers. Infection may remain localized at the site of entry or spread to other sites in the body. Most common microbes (selected examples are shown) spread through the lymphatics or bloodstream (either freely or within inflammatory cells). However, certain viruses and bacterial toxins also may travel through nerves. (Adapted from Mims CA: The pathogenesis of infectious disease, ed 4, San Diego, 1996, Academic Press.) the magnitude of the infection, the pattern of seeding, and host factors such as immune status. Sporadic bloodstream invasion by low-virulence or nonvirulent microbes (e.g., during brushing of teeth) is common but is quickly controlled by normal host defenses. By contrast, disseminated viremia, bacteremia, fungemia, or parasitemia by virulent pathogens poses a serious danger and manifests as fever, hypotension, and multiple other systemic signs and symptoms of sepsis. Massive bloodstream invasion by bacteria can be rapidly fatal, even in previously healthy individuals.
The major manifestations of infectious disease may appear at sites distant from the point of microbe entry. For example, varicella-zoster and measles viruses enter through the airways but cause rashes in the skin; polio-virus enters through the intestine but kills motor neurons to cause paralysis. Schistosoma mansoni parasites penetrate the skin but eventually localize in blood vessels of the portal system and mesentery, damaging the liver and intestine. Schistosoma hematobium also penetrates the skin, but localizes to the urinary bladder and causes cystitis. The rabies virus travels from the site of a bite by a rabid animal to the brain by retrograde transport in sensory neurons, where it then causes encephalitis and death.
Transmission depends on the hardiness of the microbe. Some microbes can survive for extended periods in dust, food, or water. Bacterial spores, protozoan cysts, and thick-shelled helminth eggs can survive in a cool and dry environment. Less hardy microorganisms must be quickly passed from person to person, often by direct contact.
For transmission of disease, the mode of exit of a microorganism from the host’s body is as important as entry into it. Every fluid or tissue that is normally secreted, excreted, or shed is used by microorganisms to leave the host for transmission to new victims.
Skin. Skin flora, such as S. aureus and dermatophytes (fungi), are shed in the desquamated skin. Some sexually transmitted pathogens are transmitted from genital skin lesions, such as HSV and Treponema pallidum (causing syphilis).
Oral secretions. Viruses that replicate in the salivary glands and are spread in saliva include mumps virus, CMV, and rabies virus.
Respiratory secretions. Viruses and bacteria can be shed in respiratory secretions during talking, coughing, and sneezing. Most respiratory pathogens, including influenza viruses, spread in large respiratory droplets, which travel no more than 3 feet. However, a few organisms, including M. tuberculosis and varicella-zoster virus, are spread from the respiratory tract in small respiratory droplets or within dust particles that can travel long distances in the air. These properties determine the type of isolation precautions that are used to prevent the spread of infection.