In order for infection and disease to occur in an individual, a process involving 6 related components must occur.
This process has been referred to as the chain of infection. The six steps or links in the chain are etiologic agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. To stop the spread of disease, one or more of these links must be broken.
1. Etiologic agent
a. Metazoan – multicellular animals many are parasites like hookworm
b. Protozoa – are single-cell organism with a well-defined nucleus eg malaria falciparum
c. Fungi - fungi are nonmotile, filamentous organism eg candidiasis
d. Bacteria – are single-celled organisms that lack a nucleus e.g. mycobacterium tuberculosis
e. Rickettsia – genus of bacteria usually in the cells of lice, ticks, fleas and mites
f. Viruses – consisting of an RNA or DNA core and outer coat of protein. Reproduce and grow in living cells like HIV, measles, mumps
g. Prions – infectious agent that do not have any genes but consist of protein with an aberrant structure replicates in animal or human tissue
* Is the usual habitat in which the agent lives and multiplies. Depending upon the agent, the reservoir may be:
i. Human - acute clinical cases / carriers
iii. Environmental – plant, soil and water
3. Portal of exit
* Is the route by which the disease agent may escape from the human or animal reservoir.
* Respiratory - airborne
* Genitourinary – sexually transmitted disease, leptospirosis
* Alimentary – by bites (mouth)
* Skin – percutaneous penetration (mosquito bites, needles)
* Transplacental – mother to fetus
4. Mode of transmission
* Is necessary to bridge the gap between the portal of exit from reservoir and the portal of entry into the host.
* Direct – contact / droplet... [continues]
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