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Overview of Infectious Diseases

Infectious diseases are illnesses caused by germs (microbes). It is important to realize that not all germs (bacteria, viruses, fungi, and parasites) cause disease. In fact, a host of bacteria normally live on the skin, eyelids, nose, and mouth and in the gut. These bacteria are called normal flora and are considered normal inhabitants. These normal flora are helpful to us! The bacteria in our bowels break down foods and form vitamin K, an essential vitamin for all of us. The normal bacteria on our skin and in our mouths protect us by preventing or decreasing the chance that we will become infected with harmful bacteria and fungi.   

The normal balance of bacteria can be upset by antibiotics and some illnesses. Viral infections often damage body surfaces and set the stage for infection by harmful bacteria.

Frequently, bacteria are present on a body surface such as the nose or throat or in the bowels, but there is no illness. This is called carriage of the bacteria, and the person with the bacteria is called a carrier. There is no illness in the carrier, but the carrier sometimes can transmit or spread the bacteria to another person. Many of the bacteria that are carried can cause infection and illness.

It is not always clear why the same strains of bacteria cause carriage in one child, mild illness in another, and serious infection in others. Sometimes it is because of factors in the child or the bacteria, but often doctors don’t understand the reasons.

Some important factors in the child include age, immunity, nutrition, genetic makeup, and general health. Newborns are at risk because their protective systems are not yet tested and are not always mature. Infants are at risk because they tend to put everything into their mouths and rarely clean their hands. Older children are less at risk because their hygiene is better and they have become immune through prior infection or carriage of bacteria.

Another important factor for a child is the use of medical devices such as catheters (tubes placed in blood vessels or into the bladder) and other tubes (e.g., from the nose to the stomach, from the nose to the lungs). These catheters and tubes provide a direct path for bacteria and fungi to get into the blood, bladder, or lungs. Medicines such as corticosteroids (used in asthma and many other conditions) and cancer chemotherapy can interfere with a child’s ability to fight infection. Even antibacterials can be a factor by killing the normal protective flora.

Factors in bacteria, viruses, and fungi include genes that determine how harmful (virulent) the microbe can be. Some germs make toxins that cause illness by themselves or contribute to infections caused by the germ. Examples include enterotoxins, which cause diarrhea; tetanus toxin, which causes lock jaw; and toxic shock toxin, which leads to low blood pressure and collapse (shock).

Infections are a normal part of childhood. Most children will have at least 6 to 8 respiratory (breathing tract) infections each year. These include colds, ear infections, sinus infections, bronchitis, and pneumonia. Infections of the bowels also are common.

When children gather together in child care settings and school, there is the opportunity for infections to spread from one child to another.

Not all infections are contagious (able to spread from person to person). Ear and bladder infections are not spread from child to child, while diarrhea and colds are easily spread.

The incubation period is the time it takes after a child is infected until he becomes ill. Sometimes the incubation is short (e.g., a day or so for the flu), while other times it is quite long (eg, 2 weeks for chickenpox and many years for human immunodeficiency virus [HIV]). In some cases, a person is contagious during the incubation period, while in others the person is not contagious until the illness begins. The amount of time a child remains contagious depends on the infection and the child. Young children are often contagious for longer than older children.

Infections are sometimes so mild that there are few or no symptoms. Other infections cause more severe illness. Infections cause harm by damaging a person’s body parts (cells and organs) and causing inflammation. Inflammation is one way a child protects himself from infection. Inflammation usually destroys the infecting agent. Unfortunately, inflammation can be harmful to the child as well. Inflammation can harm organs, cause pain, and interfere with normal body functions.

Many infections come and go with no harm to the child. Others cause pain and, sometimes, death. Some infections resolve, but leave a child with organ damage. While many germs come and go, some germs stay with your child even after the illness resolves. For example, herpesviruses (herpes simplex, cytomegalovirus, Epstein Barr virus, varicella, and human herpesvirus 6 and 7) remain in your child for a lifetime. If your child gets chickenpox, that virus stays inside his nerve cells after the rash and illness go away. The virus can reappear later in life as shingles (herpes zoster). 

Germs and Children: Terminology
normal flora Bacteria that live on or in a child
pathogen A germ that can cause a disease
colonization Presence of a germ in or on you without disease.
infection A germ causing an illness. Your body will react by making antibodies
intoxication Illness due to a toxin made by a germ
latent infection A germ (most often a virus) in a resting state
reactivation The latent germ wakes up and reproduces
carrier A child who is colonized but not sick
contagious Able to spread the illness
incubation. Time between infection and symptoms

The World of Microbes
prions Infectious proteins. The smallest known infectious agents
viruses Very small. Viruses take over your cells to reproduce themselves
bacteria Two types: free-living, normal inhabitants (normal flora); pathogens that produce disease
fungi Molds and yeasts. Fungi colonize (live on or in a child) and are pathogens
parasites Forms range from single cells (amoeba, protozoa) to worms


Last Updated
Immunizations & Infectious Diseases: An Informed Parent's Guide (Copyright © 2006 American Academy of Pediatrics)
The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.
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