An infectious disease is simply a disease resulting from infection by a pathogen. A communicable disease is a disease where the pathogen, or disease-causing agent, is transmitted from a source through direct or indirect contact to a susceptible host resulting in infection. A subset of communicable disease is contagious disease. A contagious disease is a disease that easily and rapidly spreads from a source to a susceptible host through direct or indirect contact. The infectiousness of a disease is a characteristic that refers to how many pathogen particles are needed to infect a particular host. For example, malaria is not very contagious, but it is very infectious. This means that malaria cannot be caught from a person infected with malaria. However, one bite from a mosquito carrying malaria can lead to fatal disease.
Diseases can be transmitted from any number of sources. An item is considered a source of infection if it is one in which a pathogen lives and propagates and is capable of being passed on to the environment or to other individuals. A human being can be the source of transmission during the incubation phase of a disease (before symptoms become obvious), during the active stage of a disease, or during the convalescent stage of a disease. A person can also serve as a carrier, not actively suffering from the infectious pathogen. People can be transmission sources via airborne, blood-borne, or sexual routes. The environment can serve as a transmission source through stagnant or contaminated water. Even animals can transmit disease to human beings. Common animal reservoirs include sheep, hens, cats, deer, and rats.
A reservoir of infection is a person, animal, plant, soil, or substance that harbors an infectious agent. The main living reservoir of human disease is the human body itself. A carrier is a human who harbors a pathogen and can transmit diseases to others without any sign or symptom of the diseases. The most well-known example of a carrier is Typhoid Mary. Typhoid Mary was an asymptomatic carrier of Salmonella typhi, the causative agent of typhoid. In the early 1900s, Mary worked as a cook for different families in New York City. Epidemiologists of the time were able to trace cases of typhoid back to Mary because of the time between her arrival as cook and each family's outbreak of typhoid.
A zoonosis (plural, zoonoses) is an infectious disease spread via animal reservoirs, which can be both wild and domestic animals. Zoonoses may be bacterial, viral, or parasitic. Lyme disease is an example of a zoonosis. The bacterium Borrelia burgdorferi is transmitted via salivary proteins from deer ticks to the bloodstream of the tick's meal. Of the approximately 1,400 human pathogens, 800 species are zoonotic. Of the emerging viral diseases, 70% of them are zoonoses. Well-known examples of these include West Nile virus, encephalitis, severe acute respiratory syndrome (SARS), and the Ebola virus.
Important nonliving reservoirs of infection are soil, water, and food. Soil is an important reservoir of fungal infections. Fungal infections can be transmitted by breathing or by eating soil that contains fungal spores. An example of this is the fungal infection Coccidioides in the Southwest region of the United States. People who live in this dry and dusty area of the country can inhale the spores of this fungus and develop a lung infection called Valley Fever. Symptoms range from flu-like illness to pneumonia and death.
Water is often a reservoir for gastrointestinal diseases. Contamination of water with human or animal feces allows pathogen entry into the water. In parts of India and Bangladesh, the protozoan Entamoeba histolytica present in contaminated bathing and drinking water causes amebiasis, a painful gastrointestinal infection. Cholera, caused by the bacterium Cholera vibrio, is a very dangerous disease that is spread via water. Additionally, food can be contaminated by improper storage or preparation. E. coli O157:17 outbreaks are often traced back to vegetables, such as lettuce and spinach, that are not properly cleaned prior to consumption.