The sequence of development of disease in a defined human population/community is usually characterized by a number of factors and these include:
- The disease-causing agent: This is the pathogen that caused the disease in the human/animal population. Before a disease occurs, a pathogen must first make contact and enter its human/animal host. This is called the portal of entry of the pathogen. Such a microbe must be specifically identified and linked to the infection being investigated in order to contain their deleterious effect in the community. Recognizing the cause of a disease in a community is usually the first step in unraveling the emergence and spread of a disease outbreak in a population.
- The reservoir of the pathogen: The reservoir or source is the natural environmental site from which the pathogen is being disseminated or spread to susceptible human population. It can either be human, animal or inanimate things (e.g. fomites, water, and food).Identifying and eliminating the source and reservoir of a pathogen will help to halt the spread and transmission of the disease to susceptible hosts. The reservoirs are locations in which pathogens remain viable and from which the infectivity of susceptible hosts can occur.
- Mode of disease transmission: Disease-causing agents are usually transmitted from one host to another in a variety of mechanisms. This allows the pathogen to maintain their disease in a defined human population over a long period of time if left uncontrolled. The different routes via which pathogens are transmitted from one person to another include direct body contact, water, the air, aerosols, soil, through animal vectors (insects and rodents) and fomites or inanimate objects.
- Host susceptibility: It is one thing for a pathogen to contaminate or infect a host, and it is another thing for the susceptible host to become ill by the pathogenicity/virulence of the disease-causing agent. A host with a weakened or compromised immune system will definitely be made sick when a pathogen invades. Some infectious diseases are usually warded off by the host immune system (specific or nonspecific), but in others, a combination of therapy and the immune system is needed to eradicate the disease-causing agent from the body of the host.
- Pathogen escape from the host: The entry of a pathogen is as important as its departure from the host. This is because in order to maintain its infectious disease life cycle, a pathogen must leave its already infected host and go on to infect and cause similar disease in a susceptible host. Otherwise, the disease-causing agent will not carry on its pathogenicity and virulence in a defined human population. Pathogens leave a host and go on to infect other non-infected hosts via different ways (which are usually called the portal of exit of the pathogen) but especially in areas of the host in which they are localized. For example, the pathogen responsible for causing malaria (i.e. Plasmodium parasite) leaves its human host through a blood-sucking arthropod. Other portal of exit of a pathogen include: the sputum, faeces, blood, urine or other body fluids.
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