The infections that are commonly acquired from hospitals are termed as Nosocomial infections. The hospitalization of a person causes the change in his normal flora. It gets replaced by drug resistant microorganisms typical of the hospital environment. These infections develop from the hospitals, and might show out either during the stay at the hospital or only after discharge. The infections are exogenous, as the source is from the hospital ecosystem.
    Several factors contribute to nosocomial infections;  
1. Most patients have impaired defense mechanism due to therapy being administered or due to their disease.
2. Hospital environment is heavily laden with loads of pathogens.
3. Hospital personnel, bedding, linen, utensils, acts as fomites to spread the infection.

Common types of hospital infections:
1. Wound infection; this range from wound healing or stitch abscess caused by Staphylococcus epidermidis to severely spreading disease caused by exogenous pathogens. Pseudomonas aeruginosa causes severe infections in burns.
2. Urinary tract infections; catheterization in hospitals leads to urinary tract infection. Indwelling catheters are more prone to infection. E.coli, Proteus and Pseudomonas aeruginosa. Catheterization should be done under strict aseptic conditions and indwelling catheters should be placed only if they are unavoidable and only with proper closed drainage.
3. Respiratory infections; aspiration in unconscious patients and pulmonary ventilation leads to nosocomial pneumonia. Staphylococcus aureus and gram negative bacilli are common pathogens.
4. Bacteremia; these are mainly due to the infected intravenous cannulae. An infection occurs when it is kept inserted for longer periods. Staphylococcus epidermidis bacteremia is commonly seen in patients with artificial heart valves.

Microbiology of nosocomial infections:
Streptococcus pyogenes are the most common infection causing microbe in nosocomial infections. Staphylococcus aureus belonging to phage type 80/81 and resistant to antibiotics are the commonest nosocomial cocci.Staphylococcus epidermidis and group D Streptococci also cause nosocomial infections. E.coli, Enterobacter, Klebsiella, Proteus and Serratia are also becoming common.

Drug resistant salmonellae, especially Salmonella typhimurium are also becoming prominent. Due to the ability to survive in disinfectant solutions and low temperatures Pseudomonas aeruginosa and other Pseudomonas species are also causing these infections.Hospital tetanus can occur due to the contamination of cotton by spores and improper sterilization of needles. HIV and hepatitis B &C viruses are transmitted through blood and blood products.

Viral diarrhoea, chicken pox, cytomegalovirus, herpes virus, influenza, enterovirus and arena virus can also cause hospital infections. Yeasts such as Candida albicans, molds like Aspergillus and Mucor, protozoa such as Entamoeba histolytica, Toxoplasma gondii, Plasmodia and Pneumocystis carnii also contribute to nosocomial infections.

Diagnosis and control:
1. Routine bacteriological testing of smears, culture, identification and sensitivity testing.
2. The source should be identified and eliminated when an outbreak occurs.
3. Hospital personnel, objects, water, air, etc. has to be sampled.
4. Suspected carriers should be treated.
5. Sterilization techniques have to be checked and tested. A defective autoclave may act as source and a transmitter of infections.
The control of nosocomial infections should not be a mere exercise done only during outbreaks. Strict steps have to be followed by hospital authorities in maintaining a sterile environment free from pathogens.
Proper guidelines should be given for admission, nursing and treatment of infectious patients. Such measures help to reduce the spreading of such hospital borne infections, even though complete elimination may be impossible.