Infection can be transmitted by contact, either by direct or indirect. Sexually transmitted disease like syphilis and gonorrhea illustrate spread by direct contact.
The term contagious diseases had been used for the disease transmitted by direct contact.
Indirect contact may be through the agency of forties, which are inanimate objects such as clothing, pencils or toys which may be contaminated by a pathogen from one person and act as a vehicle for its transmission to another.
Respiratory infections such as influenza and tuberculosis are transmitted by inhalation of the pathogen. Such microbes are shed by the patients into the environment, in secretions from the nose or throat during sneezing, speaking or coughing. Large drops of such secretions fall to the ground and dry there. Pathogen resistant to drying may remain viable in the dust and act as sources of infection. Small droplets, under 0.1 mm in diameter, evaporate immediately to become minute particles or droplet nuclei (usually 1 – 10 micro meter in diameter) which remain suspended in the air for long periods, acting as sources of infection.
Intestinal infections are generally acquired by the ingestion of food or drink contaminated by pathogens. Infection transmitted by ingestion may be waterborne (cholera), food borne (food poisoning) or hand borne (dysentery).
Pathogens, in some instances, may be inoculated directly into the tissues of the host. Tetanus spores implanted in deep wounds, rabies virus deposited subcutaneously by dog bite and arboviruses injected by insect vectors are examples.
Insects may work as mechanical or biological vectors of infectious diseases.
Some pathogens are able to cross the placental barrier and infect the fetus in utero. This is known as vertical transmission. This may result in abortion, miscarriage or still birth. Live infants may be born with manifestations of a disease, as in congenital syphilis. Intrauterine infection with the rubella virus. Such infections are known as teratogenic infections.
Latrogenic And Laboratory Infections
Infection may sometimes be transmitted during administration of infections, lumbar puncture and catheterisation, if meticulous care in asepsis is lacking. Modern methods of treatment such as exchange transfusion, dialysis, and organ transplant surgery have increased the possibilities for iatrogenic infections. Laboratory personnel handling infections material area at risk and special care should be taken to prevent laboratory infection.
Text Book Of Microbiology