Reservoirs of Causative Agents
The reservoir is the natural habitat of a pathogen, allowing it to survive and spread.
- Anthroponoses: Humans are the primary reservoir where pathogens are naturally maintained. Examples include tuberculosis, HIV, measles, and COVID-19, where humans are the only hosts.
Anthroponoses represent a homologous chain of infection (person-to-person transmission).
- Zoonoses: Animals serve as the primary reservoir for certain pathogens, which can spill over to humans (zoonotic transmission). Examples include rabies (dogs and bats), Avian Influenza (birds), Leptospirosis (rodents), plague (fleas on rats).
Zoonoses involve a heterologous chain of infection, where the pathogen is transmitted to humans from other vertebrates.
- Sapronoses: Environmental reservoirs such as water, soil, or decaying organic material. Examples include Legionella (water systems), Aspergillus (soil), and Clostridium tetani (soil).
Sources of Infection
The source of infection is the location from which the pathogen is transmitted.
- Primary source of infection: The location at which the pathogen is present and reproduces.
- Infected Individuals: Sick people are the most important source. Pathogens are typically excreted by the organ system through which the infection entered, with some exceptions.
- Carriers: Individuals who harbour a potential pathogen and can be a source of infection for others. Carriers may have asymptomatic infections or continue to shed the organism after recovering from the disease. There are carriers during the incubation period (typical of many viral diseases), carriers in convalescence (typical of enteric salmonelloses), chronic carriers (excrete for months/years, typical of typhoid fever), and asymptomatic carriers (carry germs on skin/mucosa without infection symptoms).
- Animal Hosts: Diseased or healthy animals that excrete pathogenic germs. Infected animals can transmit diseases to humans via bites, direct contact, or vectors.
- Secondary source of infection: Inanimate objects, materials, or third persons contributing to the transmission from the primary source.
- Environmental Sources: Pathogens can persist in the environment (soil, water, air) and infect individuals through contamination. Pathogens survive in various environments depending on temperature, humidity, and chemical factors.