Nature and Pathogenesis of Infection

Pathogen: Any organism that is capable of invading the body and cause disease

Parasite: An organism that live in another organism, deriving benefit from it but providing nothing in return.It may or may not be an pathogen. E.g E.histolytica is capable of evading the bowel wall, causing colitis and abcess in liver and other tissue BUT entamoeba. coli live in human gut without causing any disease.

Infection: Refers to a disease caused by a pathogen. It is furtehr defined by the presence of replicating organism in association with tissue damage.

Normal floral may developed into an infection. Usually, it compete with potential pathogen for attachment site by producing anti-microbial substance and compete with nutrient. Under special circumstance, it may become a pathogen, e.g C.difficile altered by antibody therapy produce toxin, causing pseudominate colitis.

Intoxification: It is DIFFERENT from infection, simply mean posioning. E.g adult botulism caused by C.botulinum, develop when food is ingested in which organism has grown and produce a neutraoxin.

Communicable disease: An infection capable of spreading from person to person.
NOTE: Not all disease are communicable.
Transmission may be by direct person to person, respiratory, sexual or mucosal contact and by insect vector.

Pathogenicity: It is the ability to cause disease E.g neisseria gonorrahoae is the causative agent. Some strain have pili while some don't. Those that lack pili are non-pathogenicity. Mechanism of pathogenicty are numerous.

Virulence: It is the power to cause severe disease. It is affected by virulence factors possessed by organism. However virulence may not alway be assoicated with pathogenicity.

Infectiousness: It is the ease which pathogen can spread in a population. E.g measle is highly infectious whereas mump is less so. It can be measure by IRR ( intrinsic reproduction rate).

The behaviour of a pathogen in a population/community depend heavily upon the interaction bewteen the host, agent and enviroment.

1) Host: It affect the chance of explosure to a pathogen and individual response to infection. E.g Travel, sexual behaviour, hygenic, occupation etc

2) Agent factor: It simply refers to the infectiousness, pathogenicity, viruluence and ability to survive in human host and under different enviroment factors. E.g the ability to produce resistance against accine, immune response etc

3) Enviroment factor: Temperature, dust and humidty, use of anitbody and pesticide affect survival of pathogen outside the host.

One case study example:

Malaria
Host factor affect the transmission of malaria. People who live in endemic area develop partial immunity as a result of repeated explosure. Sickle-cell trail individual has a low rate of parasitaemia as parasite caanot derived effective nutrition from haemglobin.

Agent of p.falciparium malaria has disease resistance to increasing range of prophylactic drug. Ths inevitable mean that traveller now have to search for new vaccine to protect themselves.

Enviroment affect transmission adversely. Malaria diesease predominate in tropical zone especially during rainy season.


Reservoir of infection: It refers to the human/animal population or enviroment in which the pathogen exist and from which it can transmitted.

1) Horizontal spread: It is between individual in the same population E.g Cough

2) Vertical spread: It is from mother to fetal. Many pathogen can cross placenta but few can cause fetal damage.

3) Zoonosis: It is animal disease which is later spread to human.

Outbreak is the occurence of a disease clearly in excess of normal expantancy.

1) Point-source outbreak: Group of individual expose to a single source of infection at a defined point of time. E.g Wedding guest who consumed contaminated food developed food poisoning shortly afterward.

2) Common source outbreak: Group of individual expose to a single source but not at a same time. E.g tattoo palour using contaminated equipment resulting in the spread of Hepatitis B.

3) Person to person outbreak: No common source but maintain by chain of transmission between infected individual. E.g measle outbreak in the school

Hopefully all these information will help you to understand the terminalogy used in M.Mic.

Ching Wei
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