Person-to-person transmission is also known as direct contact transmission and involves the direct passing of a pathogen from the source to a susceptible host. One example of direct contact transmission is horizontal disease transmission, which is the spread of pathogens between members of the same species not in a parent-offspring relationship. Examples include many infections from influenza to the common cold (adenovirus, rhinovirus, etc.).
Vertical disease transmission is the spread of pathogens from mother to offspring before or during birth. It represents another form of direct contact transmission. Mothers who are infected with HIV, toxoplasmosis, rubella, and chlamydia, for example, can transmit these diseases to their children via the placenta, during childbirth, via breast milk, or direct contact between the child and the birth canal. Zika virus is another example of vertical disease transmission. Pregnant women who become infected with the Zika virus are at risk of spreading the virus to the fetus. If this occurs, the fetus runs the risk of developing neurological defects including microcephaly, which is a small skull that leads to abnormal development of the brain.
Indirect contact transmission can occur when the agent of a disease is passed on from a reservoir to a susceptible host. This type of transmission is accomplished through a vehicle of transmission. A vehicle of transmission is an object from which pathogens can be indirectly transferred to a person via touch or ingestion. Such vehicles can include water, food, air, or fomites. A fomite is an inanimate surface or object that carries pathogenic microbes. Fomites can be towels, bedding, money, handkerchiefs, drinking cups, or any nonliving object on which a disease can live until transferred to a viable host. Droplet transmission is the spread of microbes through droplet nuclei in air. Droplet nuclei are the result of the evaporation of small water droplets that surrounded the pathogen. Once the water has evaporated, the microbe is suspended in the air for several hours. These droplets can be put into the air by coughing, sneezing, laughing, and even talking. Some disease examples of droplet transmission spread are pneumonia, influenza, and pertussis (whooping cough).
Waterborne transmission causes pathogens to be spread by water contaminated with untreated or poorly treated sewage. In the aftermath of a major flooding event, an outbreak of cholera often occurs. The causative agent Cholera vibrio is found in feces. If feces-contaminated water is consumed, the bacteria can infect the small intestine of the host, which causes severe vomiting and diarrhea. Foodborne pathogens can be spread by foods that are poorly refrigerated, incompletely cooked, or cooked under unsanitary conditions. Bacteria in the genus Salmonella commonly contaminate foods such as beef, poultry, milk, eggs, fruits, or vegetables, causing outbreaks of illness.
A vector is an organism that serves as a carrier of a pathogen between hosts with the pathogen having minimal effect on the fitness of the carrier. A mechanical vector is an animal carrier of a pathogen between hosts. For example, a fly that lands on road carrion may transmit bacteria from the carrion to food left out at a picnic. Picnic goers eating this food may become sick from the bacteria. Cats are another example of mechanical vectors. They carry the parasite Toxoplasmosis gondii, which can infect humans if the T. gondii deposited in cat feces are ingested. A biological vector is an organism within which a pathogen reproduces and then the vector transmits the pathogen by biting a host. Arthropods are a very important group of biological vectors. Lyme disease is caused by the bacterium Borrelia burgdorferi, which inhabits the salivary glands of ticks. A bite from an infected tick allows the bacteria to enter the bloodstream of the mammal host. The Center for Disease Control (CDC) reports that 300,000 people contract Lyme disease in the U.S. each year. Lyme disease can result in long-term symptoms, including arthritis, heart palpitations, intermittent pain, and problems with long-term memory. These symptoms are most common if a patient delays treatment. While the mortality rate of Lyme disease is low, many diseases can be quite harmful without being fatal. Non-arthropod biological vectors include mammals such as foxes and raccoons that transmit rabies to humans via a bite and domestic poultry that transmit avian influenza virus A shed in bird feces or saliva.