Infectious disease is disease caused by a pathogen which enters the body and triggers the development of an infection. These diseases have a range of causes, and they can be found all over the world. These diseases are considered contagious or communicable, meaning that they can be passed from person to person. It is also possible for such diseases to spread indirectly through unhygienic conditions, or from animals to people, in which case they are known as zoonotic diseases.
A variety of pathogens can be responsible for infectious disease, including viruses, bacteria, fungi, protozoans, and prions. Within these large categories of infectious organisms, there are numerous modes of transmission and a colorful assortment of symptoms, although surprisingly few organisms cause disease, when one considers the diversity of viral, bacterial, fungal, and protozoan life. In order to treat an infectious disease, doctors must be able to knock out the source of the infection and repair the damage it has done to the body. Many of these diseases make the body vulnerable to secondary infections, in which other organisms move in to take advantage of a weakened immune system, and this can be very problematic.
The study of infectious disease is known as epidemiology. Epidemiologists work to determine the source of the disease so that they can develop new treatment approaches. They also identify emerging outbreaks, which may develop into epidemics or pandemics, and areas where a disease is endemic, meaning that it occurs regularly. Malaria, for example, is endemic to some regions of Africa and Southeast Asia.
There are a variety of techniques which can be used to prevent the spread of infectious disease. Basic hygiene eliminates many organisms, as long as people wash their hands, use clean drinking water, and have access to clean medical facilities. Reduction of contact with vectors of zoonotic transmission such as insects and rodents can also reduce the incidence, as can education in communities where a particular disease is endemic.
Antivirals, antibacterials, and antifungals are all used in the fight against infectious disease, sometimes prophylactically to prevent infection in endemic areas. Doctors also use a variety of medical tests and screening tools to identify patients and at-risk populations, and additional medical treatments such surgery and minor procedures are also used in the treatment and prevention of infectious diseases.
Worldwide, infectious disease is a common killer, especially in developing nations. Respiratory infections are the deadliest diseases, followed by HIV/AIDS, diarrheal diseases, tuberculosis, and malaria. Many of these conditions are fully preventable with minimal effort, making the high loss of life to things like diarrheal diseases in some regions of the world especially unfortunate.