Infection Definition Classification And Method of transmission
It involves interaction between the host and the infecting microorganism. Based on their relationship to the host microorganism can be classified as saprophytes & parasites.
Saprophytes:- The word came from greek papers mean decayed & phyton means plant. They are free-living microbes that maintain on dead or decaying organic matter. They are found in soil and water. Play an important role in the degradation of organic matter.
Parasites:- They can either be pathogen greek pathos means suffering & gen means disease-producing or commensals come from Latin com means with & mensa means table i.e living together. They can establish themselves and multiply in hosts.
The term infection & infectious disease is different. A place (lodgement) & multiplication of a parasite in or on the tissue of a host constitutes infection.
Classification of infection
1) Initial infection with a parasite in a host is called PRIMARY INFECTION.
2) Subsequent infections by the same parasite in the host are called as REINFECTIONS.
3) When a new parasite sets up an infection in a host whose resistance is lowered by preexisting infection is called SECONDARY INFECTION.
4) When a patient already suffering from a disease a new infection is set up from another host or other source is called CROSS INFECTION.
5) Cross infections occurring in hospitals are called as NOSOCOMIAL INFECTION.
6) Depending on whether the source of infection is within or outside the host’s own body, infection is classified as ENDOGENOUS or EXOGENOUS.
Apart from these basic terms mention above, some other infections are iatrogenic infection, inapparent infection, atypical infection, latent infection.
Sources of infection
The commonest source of infection in humans are they. It could be patients or carriers. The carriers can be healthy, convalescent. Depending on the duration carriers can be temporary & chronic. The temporary carrier lasts less than 6 months, while chronic last several year & sometimes rest of one’s life.
Many pathogens can infect both humans and animals. Therefore animals may act as a source of human infection. The infection in animals may be asymptomatic. They can serve to maintain the parasite in nature & act as a reservoir of human infections. They are called as RESERVOIR HOST.
Bloodsucking insects may transmit the pathogen to humans and cause a disease called as ARTHROPOD –BRONE DISEASE.
Soil & water
Some pathogens can survive in the soil for very long periods spores of tetanus bacilli may remain viable in the soil for several decades & serve as a source of infection. Water may act as a source of infection due to contamination with pathogenic microbes such as cholera vibrio etc.
Again contaminated food may act as a source of infection. The presence of the pathogen in food may be due to external contamination (staphylococcus) or pre-existing infections such as meat or other animal products (Salmonella spp).
Method of transmission of infection
- Contact:- Infection acquired by contact, which may be direct or indirect. The term contagious disease had been used for diseases transmitted by direct contact from infectious disease. Indirect contact may be through the agency of families, which are inanimate objects such as clothing, pencils may be contaminated by the pathogen.
- Inhalation:- Respiratory infection such as influenza & tuberculosis is transmitted by inhalation of the pathogen such infection are shed by the patients into the environment during sneezing, coughing, or speaking in the form of a droplet.
- Ingestion:- Generally acquired by the ingestion of food or drink contaminated with the pathogen. It can be waterborne (cholera). Foodborne (food poisoning) or hand borne (dysentery).
- Inoculation:- Pathogen in some instances may be inoculated directly into the tissue of the host. Inoculation may be iatrogenic when unsterile syringes & surgical equipment are employed.
- Congenital:- Some pathogens can cross the placental barrier & infect the fetus. This is known as vertical transmission.
- Iatrogenic & laboratory infection:- Infection may sometimes be transmitted during the administration of infection. Modern methods of treatment such as transfusion, organ transplant surgery have increased the possibilities for such infection.
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