Infectious diseases. Etiology & pathogenesis. Classification according to organ site and transmission mechanism
Various categories of infectious agents:
- Prions - abnormal forms of host protein that cause transmissible spongiform encephalopathies (including Creutzfeldt-Jakob disease)
- Viruses - obligate intracellular parasites that use host cell for replication
- Bacteria - prokaryotic single-celled organisms
- Fungi - eukaryotes with thick chitin containing cell walls, invasive infections mainly affect those who are immunocompromised
- Protozoa - single celled eukaryotes that are a major cause of disease in developing countries. They replicate intracellular (e.g. plasmodium, leishmania) or extracellular (e.g. trichomonas vaginalis)
- Helminths - parasitic worms, three groups: nematodes, cestodes and trematodes (aka roundworms, tapeworms and flukes).
- Ectoparasite - insects (e.g. lice) or arachnids (e.g. ticks) that attach and can transmit bacteria (e.g. Borrelia burgdorferi, agent of lyme disease from deer ticks)
Infectious agents can get into the body in a number of different ways:
- Via the skin (may be opportunistic e.g. S.aureus) often have to break through the skin via an existing wound or vai an injection etc
- Via the GI tract - can be absorbed or release toxins that disrupt the normal GI state
- S.aureus, V.cholerae, shigella and salmonella typhi all good examples
- Via respiratory tract - many microorganisms - e.g. influenza virus
- Via urogenital tract - bacteria ascend through the urethra
Once inside the body microorganisms can proliferate locally, at the site of infection , or spread to distant sites by lymphatics, blood or nerves.
After transport, organisms look to be released and transmitted onwards, this may be by shedding in the skin, release in saliva, via respiration, shedding in stool, exit via blood, urine, STIs or vertical transmission to fetus.
The mechanism of injury is dependent on the nature of the infectious agent, below we look at some examples:
- Mechanism of viral injury
- Replicate at host cells expense, effectively kill the cell based on one of a number of mechanisms: direct cytopathic effects, antiviral immune response or transformation of infected cells.
- How the viral injury manifests depends a great deal on virulence, immune response and viral cell tropism
- Mechanism of bacterial injury
- This may be adhesion to host cell, invasion of host cell forcing cellular lysis (or just as transport)
- By bacterial toxins (these can be endotoxins or exotoxins - endotoxins are elements of the bacteria themselves, exotoxins are secreted)