Infectious diseases may occur sporadically in single individuals, in localized groups, and in epidemics or in worldwide pandemics. Certain infections are endemic to an area, consistently occurring in that population. Knowledge of the modes of transmission of microorganisms and of methods of control is essential for the prevention and control of infection.
1. Transmission of Infectious Agents
A chain of events occurs during the transmission of infecting organisms from one person to another. The reservoir, or source of infection, maybe a person with an obvious active infection in an acute stage, or a person who is asymptomatic and shows no clinical signs or symptoms. The latter may be in the early incubation stage of infection, or the person may be a carrier of the organism and never develop an infection. Hepatitis B is an example of an infection that is often transmitted by unknown carriers or persons who have a subclinical form of infection that is very mild, with few or no manifestations. The reservoir may also be an animal or contaminated water, soil, food, or equipment.
The mode of transmission from the reservoir to the new host maybe:
• Direct contact with no intermediary, such as touching an infectious lesion or sexual intercourse. Microbes may be in the blood, body secretions, or lesion. Not all microorganisms can cross the blood-brain barrier or the placental barrier. However, some microbes that can cross the placenta have disastrous effects on the fetus. For example, Treponema pallidum, the cause of syphilis, can lead to multiple defects or death in the fetus, and toxoplasma gondii, the cause of toxoplasmosis, results in many neurological deficits;
• Indirect contact involving an intermediary such as a contaminated hand or food, or a fomite, an inanimate object such as facial tissue or bed linen that carries organisms. In some cases, there are several stages in transmission. For example, shellfish can be contaminated by human feces in the water. The microorganisms in the shellfish are then ingested and cause infection in another human;
• Droplet transmission (oral or respiratory) occurring when respiratory or salivary secretions containing pathogens such as influenza viruses or tuberculosis bacteria are expelled from the body. The organisms from these secretions may be inhaled directly by another person close by, or fall on nearby objects to be transmitted indirectly, or they may remain airborne for a time; and
• Vector-borne, when an insect or animal serves as an intermediary in a disease such as malaria.
Hands are considered a major culprit in spreading infection from many sources, in health care facilities, the home, office, or school. Frequent, proper handwashing is essential in infection control.
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Nosocomial infections occur in health care facilities, including hospitals, nursing homes, doctors’ offices, and dental offices. CDC estimates 10% to 15% of patients acquire an infection in the hospital. Reasons for these infections include the presence of many microorganisms in these settings, patients with contagious diseases, overcrowding, use of contaminated instruments, the chain of transmission through staff, diagnostic procedures and equipment, therapeutic aids, and food trays. Often the patient’s health status is compromised in some way, by age or by weakened immune systems or other illness or surgery. Also, many microbes in healthcare settings are resistant to several drugs. Common Nosocomial infections include urinary tract infections (the highest number), pneumonia, diarrhea, and surgical wound infection. Most infections in health care facilities are spread by direct contact between persons or by contaminated objects. Recently there have been several outbreaks of infection in hospitals by a more dangerous strain of the bacterium, Clostridium Difficile, particularly in intensive care units where most individuals are taking antimicrobial drugs. The resulting disruption of the normal flora allows Clostridium Difficile to multiply and cause severe diarrhea and many deaths.
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