Infection

Infection is defined as "the presence and growth of a microorganism that produces tissue death". It is the prsence of certain microorganisms which predispose us to the possibility of an infection.

As humans, we are continuously exposed to a variety of extremely small life forms known as microorganisms or microbes. These microbes are everywhere. They are in the air we breathe, on the clothes that we wear, thay are on our skin and even inside our mouth and intestines.

MICROBES

Microbes capable of causing infection that result in diseases are called pathogens. Degree of pathogenicity of a microbe is referred to as its 'Virulence'.
Virulence is a quantitative term . Infection of a susceptible animal with a highly virulent pathogen will almost always result in diseases, whereas infection witha pathogen of low virulence will often fail to produce disease, and will only produce a mild disease.

There are many microbes that cannot produce even infection, let alone diseases. The great bulk of free living microorganisms are of this type. some free living microorganisms can, under some circumstances, cause infection and diseases.
Normally freeliving microbes that can opportunistically cause infection are called facultative parasites.

Certain microbes assist humans in maintaining a particular body process. for instance, in the digestive system of humans there lives a class of microorganisms responsible for breaking down undigested foo and producinfg specific vitamins.
Some cause disease in healthy individuals in the absence of specific immunity, eg. Staphylococcus aureus, Salmonella sp., Shigella sp., Corynbacterium diphtheria, Mycobacterium tuberculosis, Bordetella pertussis, Hepatitis A and B viruses, Rubella virus, HIV etc.

Some cause disease, other than trivial local infections , only in persons with reduced resistance of infection including new born infants or when implanted directly into tissueor a normally sterile body area eg. Streptococcus agalactiae, Enterococcus sp., Clostridium tetani, Escherichia coli, Klebsiella sp., Candida sp. etc.

Microbes are also found living in the skin, in the urinary tract and respiratory tract. An infection can occur when when a harmful microorganism (pathogen) invades a human. This human becomes the microbe's host. Whether the pathogen's host develops and infection or disease or not depends on two factors:
1) The The host's immune system's ability to defend itself through containment or
2) destruction of the microbes and the virulence of the microorganism.

SOURCES OF INFECTION

The source of causative infecting microorganism may be exogenous - from another patient or a member of the hospital or the inanimate environment in the hospital.
It may be endogenous from the patient's own flora which at the time of infection may invade the patient's tissue spontaneously or be introduced into them by surgical operation, instrumental manipulation and nursing procedures.

The microorganism can be transmitted into the human directly or indirectly.-

Direct Trasmission occurs through:
1. Transplacental and congenital
2. Contact with man, animal or inanimate objects.
3. Droplet infection.
4. breach of skin or mucous membrane.
 
Indirect transmission is possible by any of the following mechanisms:
1. Vehicles (water, food, milk etc.)
2. Vectors (mechanical or biological)
3. Fomites
4. Unclean hand and finger.

HOW DO PATHOGENS SPREAD

Pathogens which produce infections or diseases, spread to humans and animals through a series of steps known collectively as the chain of infection.
A communicable infection is one that is easily passed from one person to another.

Whether or not a tissue will develop an infection after the contamination depends upon the interaction between the contaminating organisms and the host.
Health individuals have a normal general resistance to infection. Patients with underlying disease, newborn babies, and elderly have less resistance and will probably develop an infection after contamination.

Following are the ways by which pathogens spread in a hospital:
1. Air borne
2. Infection by contact
3. Infection acquired from food
4. Infection associated with water
5. Infection from hospital equipment
6. Infection by inoculation