Communicable diseases are transmitted among hosts through a wide range of actions and routes. There is also a range in the capacity for pathogens to spread among hosts and those that spread easily are considered contagious. Transmission can occur through direct or indirect contact with other organisms that harbor the infectious agent or with inanimate objects where the infectious agent resides.
Forms of direct contact include vertical transmission, which is the spread of pathogens from mother to offspring before or during birth. HIV and cytomegalovirus, or CMV, are two examples of vertically transmissible pathogens. This is in contrast to horizontal transmission, which is the spread of pathogens between members of the same species not in a parent-offspring relationship. Other forms of direct contact include physical touching and sexual contact that spread pathogens such as the herpesviruses and Chlamydia trachomatis, and droplet transmission, where a pathogen is spread suspended in droplets of water in air. This is commonly seen when an infected host sneezes and mucoid droplets from the nasopharynx carry, for example, rhinovirus or cytomegalovirus particles in the air to be inhaled by the next potential host.
Transmission of communicable diseases by an indirect route can occur through airborne transmission, such as inhalation of fungal spores; fecal-oral transmission, through which the pathogen is shed in host feces and contaminates food or water consumed by the next host; contamination of an intermediate object, such as nonliving matter, or fomites (for example, a doorknob or telephone that everyone in the office uses); and vector-borne transmission when pathogens are spread via disease vectors, or animals that move the pathogen from host to host and transmit pathogens.
There are many arthropod vectors, including mosquitoes that transmit communicable diseases such as dengue, malaria, West Nile virus, and yellow fever. Ticks, particularly the common Ixodes scapularis, or deer tick, carry Borrelia burgdorferi, the causative agent of Lyme disease. Other diseases carried by ticks include babesiosis, Rocky Mountain spotted fever, and ehrlichiosis. Rats are often credited as the cause of the bubonic plague, one of the most devastating diseases of human history, killing 75 to 200 million people in the 14th century. However, it was an insect vector, Xenopsylla cheopis—the rat flea—that carried the bacterium Yersinia pestis, which resulted in the plague. There are nonarthropod vectors, too. These include mammalian vectors such as bats, raccoons, coyotes, and foxes that can transmit rabies. The reservoir of a pathogen is any animal, person, or substance in which that pathogen normally lives and multiplies. For example, the reservoir for Borrelia burgdorferi is a variety of mammals, including deer, western and eastern gray squirrels, shrews, and chipmunks. The reservoirs for malaria are human beings. The reservoirs for West Nile virus are "old-world" bird species.
Contamination is the presence of an outside organism in or on nonliving matter. Contamination of air can occur when a pathogen becomes aerosolized and remains floating—adenovirus after a person with an upper respiratory infection sneezes, for example. Inanimate surfaces such as doorknobs can become contaminated by contact with infected persons. Studies have found that most viruses survive on a doorknob for approximately 24 hours, whereas certain bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), can live on a doorknob for up to five months or more.
Food can be contaminated during processing and handling. This can occur during the slaughter of the animal, where pathogens from the animal's hide get into the final meat product. If pathogens are present on the surfaces where the meat is processed, such as in storage bins or conveyor belts, the meat becomes contaminated. Meats may also come from infected animals. For example, pigs can become infected with the roundworm parasite Trichinella spiralis by eating the Trichinella cysts in raw meat. A human who eats undercooked pork will also ingest the Trichinella cysts, which open and release their larvae in the stomach. The larvae travel to the intestines, where they parasitize their human host. They also travel through the bloodstream to various organs and to the skeletal muscles, where they encyst. These encysted larvae can remain in this state for the life of the host.Water and soil are typically contaminated by pathogens that are able to survive outside the host for extended periods when passed through feces. This indirect transmission process is called the fecal-oral route. Many pathogens are released from the body in feces. The feces can then contaminate water, soil, or food, or the infected person spreads microbes if proper hygiene is not practiced. A new host then ingests the contaminated material. Fecal-oral infections include diarrhea caused by Shigella species, typhoid fever caused by Salmonella typhi, gastroenteritis from the causative agent Norovirus, and giardiasis from the protozoan Giardia. Risk factors for fecal-oral contamination include poor sanitation, inadequate safe water supply, and poor hygiene.