Understanding the mode of transmission of infectious agents is extremely important from a public health perspective. Interrupting the chain of transmission is an excellent way to prevent infectious diseases. The mode of transmission of many infectious diseases is “human-to-human,” but infectious diseases are also transmitted from nonhuman sources such as soil, water, and animals. Fomites are inanimate objects, such as towels, that serve as a source of microorganisms that can cause infectious diseases. Table1 describes some important examples of these modes of transmission.
Table1. Important Modes of Transmission
Most infections are acquired by transmission from external sources. Pathogens exit the infected patient most frequently from the respiratory and gastrointestinal tracts; hence, trans mission to the new host usually occurs via airborne respiratory droplets or fecal contamination of food and water. Organisms can also be transmitted by sexual contact, urine, skin contact, blood transfusions, contaminated needles, or biting insects. The transfer of blood, either by transfusion or by sharing needles during intravenous drug use, can transmit various bacterial and viral pathogens. The screening of donated blood for Treponema pallidum (the cause of syphilis), human immunodeficiency virus (HIV), human T-cell lymphotropic virus, hepatitis B virus, hepatitis C virus, and West Nile virus has greatly reduced the risk of infection by these organisms.
The major bacterial diseases transmitted by ticks in the United States are Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, relapsing fever, and tularemia, with Lyme disease being the most common. Ticks of the genus Ixodes (deer tick) transmit three infectious diseases: Lyme disease, ehrlichiosis, and babesiosis, a protozoan disease. Dermacentor ticks (dog tick) transmit several diseases: Rocky Mountain spotted fever, tularemia, ehrlichiosis, anaplasmosis, and tick paralysis.
Bacteria, viruses, and other microbes can also transmit from parent to progeny via vertical transmission. This includes transmission across the placenta, within the birth canal during birth, and via breast milk. Table 2 describes some medically important organisms that are transmitted vertically. The four most important portals of entry are the respiratory tract, gastrointestinal tract, genital tract, and skin (Table3). Important microorganisms and diseases transmitted by water are described in Table4, by food are listed in Table 5, and those transmitted by insects are listed in Table 6. The specific mode of transmission of each organism is described in the subsequent section devoted to that organism.
Table2. Vertical Transmission of Some Important Pathogens
Table3. Portals of Entry of Some Common Pathogens
Table4. Transmission of Important Waterborne Diseases
Table5. Bacterial Diseases Transmitted by Foods
Table6. Bacterial Diseases Transmitted by Insects
Animals are an important source of organisms that infect humans and can serve as the source (reservoir) or the mode of transmission (vector). These diseases are referred to as zoonoses, many of which are listed in Table 7.
Table7. Zoonotic Diseases Caused by Bacteria
