The three major types of reservoir associated with infectious disease are humans, other animals, and nonliving reservoirs. Human reservoirs are in infected people who are either sick from their infection, are infectious prior to developing symptoms, or are asymptomatic carriers. Asymptomatic carriers can transmit diseases such as AIDS, diphtheria, and typhoid fever, readily infecting others but without showing any signs of disease.
Non-human animals form the second major type of reservoir. Diseases that are transferred from animal to human are called zoonotic diseases and these usually occur following direct contact with the animal. However, indirect contact can also transmit pathogens from animal to person such as through contact with the waste material of a litter box, fur, feathers, infected meats, or vectors.
Nonliving reservoirs include water, food, and soil. Water may be the most dangerous of these, with many diseases spread from this reservoir and these diseases being endemic in many parts of the world. Poor sanitation, as seen in many underdeveloped countries, results in fecal contamination of water, and as water is essential for life it becomes a key component in the fecal-oral route of transmission. In the case of food acting as a pathogen reservoir, contamination is part of the naturally occurring spoilage of food.
Predisposing factors, other than the overall health of the host, which can influence the disease process include age, gender, lifestyle, occupation, emotional state, and climate. Age affects the overall health of the host. In particular, a decline in defensive capability occurs with increasing age and translates to an increase in disease with age. In addition, young children and babies are also more susceptible as they have less well developed immune systems. Gender bias can also be seen in certain infections such as a greater prevalence of urinary tract infections in females and respiratory infections in males. Lifestyle and occupation can also lead to a predisposition to infection, and emotional state may also play a role in susceptibility to infection. Climate can also be a predisposing factor, for example, affecting the variety of infectious organisms present in a given location, the association between colder climates and a greater incidence in respiratory infections, and warmer climates allowing for longer periods when disease-transmitting vectors are present.
Burn patients, those undergoing chemotherapy, and transplant patients may all be considered to be more susceptible and have a higher potential for contracting infectious diseases. The loss of large areas of skin, the primary physical barrier to infection, in burn patients results in a greater chance of septicemia. Infection with Pseudomonas aeruginosa is also a major problem for burn patients as this organism is very resistant to the antiseptics and disinfectants which are used to control bacterial growth, and is also becoming resistant to antibiotics. Patients undergoing chemotherapy often suffer from profound neutropenia as a side effect of the drugs used, and are rendered temporarily immune-incompetent. The infections associated with neutropenia are primarily bacterial and fungal, and the chance of infection during the administration of these drugs is very high. Transplant patients are placed on drug regimens that diminish their overall immune capability to reduce their immune response against the transplanted organ and prevent rejection. Although this reduces the chance of rejection it also makes the patient more susceptible to infection.