User:TooManyOptions/Disease ecology

Disease ecology is a sub-discipline of ecology concerned with the mechanisms, patterns, and effects of host-pathogen interactions, particularly focusing on infectious diseases through the lens of environmental factors and the affected communities. Disease ecology operates under the assumption that host-pathogen interactions can be conceptually united with other interspecific interactions, such as predation or parasitism.

While the field is often defined within the context of ecology as a whole, disease ecology relates ideas from a wide variety of medical and biological disciplines including immunology, epidemiology, and genetics.

Parasitism
Disease ecology views infections as parasitism, considering bacterial and viral infections as microparasites. Parasitism in ecology is important because it can shape the way many habitats function. Parasites can alter the timings of ecological events, such as biogeochemical cycles, and can shift the flow of energy in a habitat. Parasites are able to limit population growth, which may lead to a shift in the balance of an ecosystem. Another way parasites impact systems is through nutrient cycles. Parasites are able to alter the distribution of nutrients in a system through the relationship they have with a host and the host’s diet. Parasites are capable of such a significant impact because they are well specialized to infect their hosts. Ideally for the parasite, the host remains alive and able to sustain the parasite until it reproduces but parasite aggregation and diseases can cause more damage to a host than is sustainable. Because most microparasites are specialized to specific hosts, diseases that are specialized for the same hosts use those parasites as vectors, which can cause further stress on the host.

Parasite aggregation
Most individual parasites cause a negligible effect on their hosts, but a host infected with tens or hundreds of parasites can be severely inhibited by the stress. Parasites are not evenly distributed among their hosts. The 80:20 rule, also known as the Pareto Principle, applies to parasite aggregation in hosts. On average, 80% of the parasite load is distributed among 20% of the hosts.

In relation to predator-prey interaction
When a host has become encumbered by the stress of its parasites, it becomes easier to prey upon. Predators often will prefer sick or infected prey because of the opportunity weak prey presents. Without the presence of a predator species the prey species could exceed manageable numbers, leading to the rapid spread of pathogens throughout the prey population. However, predator feeding can also disturb a pathogen that was previously dormant, leading to an outbreak that otherwise would not have occurred. Some parasites are able to survive when their host species is consumed, leading to the parasite being distributed in the waste of the predator which can continue the spread of disease.

SIR Model
Found in disease ecology as well as epidemiology, SIR models are used as a simple way to model how a disease that confers immunity after infection. SIR, categorizes individuals into three labels based upon their interaction with the disease: Susceptible, Infected, or Recovered. Susceptible individuals have never come into contact with the disease and hold no immunity. Infected individuals are currently contagious and able to spread the disease to any susceptible individual they contact. Recovered individuals are those who had the disease, survived, and retain immunity. By altering the infectious period, infectivity of the disease, and death rates the SIR model can model outbreaks as function over time.

Climate change*
As climate change continues to disrupt ecosystems around the world it makes both human and non-human populations more vulnerable to disease. The subject is increasingly attracting the attention of health professionals and climate-change scientists, particularly with respect to malaria and other vector-transmitted human diseases. As climates change, diseases vectors ranges are increasing, allowing diseases to reach new territories. This is an important, novel field that requires further research.