Infectious disease is a disease caused by a pathogen that enters the body and triggers the development of an infection. Infectious diseases have a variety of causes, and can be found throughout the world. These diseases are considered contagious or communicable, meaning they can be passed from person to person. It is also possible that such diseases 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 diseases, including viruses, bacteria, fungi, protozoa, and prions. Within these broad categories of infectious organisms, there are numerous modes of transmission and a wide variety of symptoms, yet surprisingly few organisms cause infectious disease, when considering the diversity of viral, bacterial, fungal, and protozoal life. To treat an infectious disease, doctors must be able to eliminate the source of the infection and repair the damage it has caused to the body. Many infectious diseases make the body vulnerable to secondary infections, where other organisms move in to take advantage of a weakened immune system, and this can be very problematic.
The study of infectious diseases is known as epidemiology. Epidemiologists work to determine the source of an infectious disease so they can develop new treatment approaches. They also identify emerging outbreaks, which may become epidemics or pandemics, and areas where a disease is endemic, meaning it occurs regularly. Malaria, for example, is endemic in parts of Africa and Southeast Asia.
There are a variety of techniques that can be used to prevent the spread of infectious diseases. Basic hygiene kills many organisms, as long as people wash their hands, use clean water, and have access to clean medical facilities. Reducing contact with vectors of zoonotic transmission, such as insects and rodents, can also reduce incidence, as can education in communities where a particular infectious disease is endemic.
Antivirals, antibacterials, and antifungals are used in the fight against infectious diseases, sometimes prophylactically to prevent infection in endemic areas. Physicians also use a variety of medical tests and screening tools to identify patients and populations at risk, and additional medical treatments, such as surgery and minor procedures, are also used in the treatment and prevention of infectious diseases.
Around the world, infectious diseases are a common killer, especially in developing countries. Respiratory infections are the deadliest infectious diseases, followed by HIV/AIDS, diarrheal diseases, tuberculosis and malaria. Many infectious diseases are completely preventable with minimal effort, making the huge loss of life to diseases like diarrhea in some regions of the world especially unfortunate.