Infectious diseases are caused by germs, such as viruses, bacteria, and parasites. Contagious or communicable diseases are those that can be spread from one person to another. Infectious diseases that commonly occur among children are often communicable or contagious and may spread very easily from person to person.

Infants and toddlers are highly susceptible to contagious diseases. They have not yet been exposed to many of the most common germs; therefore, they have not yet built up resistance or immunity to them. Also, young children have many habits that promote the spread of germs. For example, they often put their fingers and other objects in their mouths. In this way, germs enter and leave the body and can then infect the child or be passed on to others.

In order for germs to be spread from one person to another, three things must happen:

1. Germs must be present in the environment. A person may carry the germs; or their infectious body fluids, such as discharge from the eye, nose, mouth, or digestive (gastrointestinal) tract, may travel through the air and/or land on a surface.
2. A person who is not immune to the germ must come in contact with or be exposed to the germs.
3. The contact or exposure must be in a way that leads to infection.

As the table shows,

• Skin infections may be spread by touching fluid from another person’s infected sores.
• Respiratory tract infections with symptoms such as coughs, sneezes, and runny noses are spread mainly through exposure to fluids present in or expelled from another person’s mouth and throat (saliva or mucus), often when an uninfected person touches these discharges with their hands and then touches their mouth, eyes, or nose.
• Intestinal tract infections, including some types of diarrhea, usually are spread through exposure to germs in the feces. Many of the germs discussed in this manual are spread through what is known as “fecal-oral” transmission. This means that germs leave the body of the infected person in the feces (poop) and enter the body of another person through the mouth. In most situations, this happens when objects (including toys, fingers, or hands) which have become contaminated with undetectable amounts of feces are placed in the mouth. Fecal-oral transmission can also occur if food or water is contaminated with undetectable amounts of human or animal feces, and then is eaten or drunk. Improperly prepared foods made from animals (for example, meat, milk, and eggs) are often the source of infection with Campylobacter, E.coli O157, and Salmonella.  
• Some infections, like infection with Salmonella and Campylobacter, may be spread through direct exposure to infected animals.
• Blood infections are spread when blood (and sometimes other body fluids) from a person with an infection gets into the bloodstream of an uninfected person. This can happen when infected blood or body fluid enters the body of an uninfected person through cuts or openings in the skin; the mucous membrane that lines body cavities, such as the nose and eye; or directly into the bloodstream, as with a needle.

  Some diseases, such as chickenpox, impetigo, and hand-foot-and mouth disease, can have more than one transmission route. For example, they may be spread through air or by direct contact with the infectious germ.