Since the start of widespread vaccinations in the United States, the number of cases of formerly common childhood illnesses like measles and diphtheria have declined dramatically. Immunizations have protected millions of kids from potentially deadly diseases and saved thousands of lives.

In fact, certain diseases crop up so rarely now that parents sometimes ask if vaccines are even necessary anymore. This is just one common misconception about immunizations. The truth is, most diseases that can be prevented by vaccines still exist in the world, even in the United States, although they occur rarely.

The reality is that vaccinations still play a crucial role in keeping kids healthy. Unfortunately, misinformation about vaccines could make some parents decide not to immunize their children, putting them and others at a greater risk for illness.

To better understand the benefits and risks of vaccines, read on for answers to some common questions.

What do immunizations do?

Vaccines work by preparing a child's body to fight illness. Each immunization contains either a dead or a weakened germ (or parts of it) that causes a particular disease.

The body practices fighting the disease by making antibodies that recognize specific parts of that germ. This permanent or longstanding response means that if someone is ever exposed to the actual disease, the antibodies are already in place and the body knows how to combat it and the person doesn't get sick. This is called immunity.

Will my child's immune system be weaker by relying on a vaccine?

No, the immune system makes antibodies against a germ, like the chickenpox virus, whether it encounters it naturally or is exposed to it through a vaccine. Being vaccinated against one disease does not weaken the immune response to another disease.

Will the immunization give someone the very disease it's supposed to prevent?

This is one of the most common concerns about vaccines. However, it's impossible to get the disease from any vaccine made with dead (killed) bacteria or viruses or just part of the bacteria or virus.

Only those immunizations made from weakened (also called attenuated) live viruses - like the chickenpox (varicella) or measles-mumps-rubella (MMR) vaccine - could possibly make a child develop a mild form of the disease, but it's almost always much less severe than the illness that occurs when someone is infected with the disease-causing virus itself. However, for kids with weakened immune systems, such as those being treated for cancer, these vaccines may cause problems.

The risk of disease from vaccination is extremely small. One live virus vaccine that's no longer used in the United States is the oral polio vaccine (OPV). The success of the polio vaccination program has made it possible to replace the live virus vaccine with a killed virus form known as the inactivated polio vaccine (IPV). This change has completely eliminated the possibility of polio disease being caused by immunization in the United States.

Read more answers to common immunization questions.