Would you like to protect your child from some of the deadliest diseases in history? I'm sure you would. That's what vaccines do.

But they also do a lot more than that. They protect your neighbors' kids and other children around the country at the same time; ultimately, if everyone follows suit, they'll rid the entire world of diseases that have been crippling and killing children for centuries.

Vaccines are probably the most powerful health tools ever developed.

To understand why, you have to know a little bit about how your immune system works. Your immune stystem is an incredible defense mechanism. Whenever a virus invades your body, your immune system produces proteins called antibodies. These antibodies hunt down and destroy the viruses that make you sick. Since it takes your immune system a while to produce these antibodies, the first time a specific invader strikes, you'll get sick until the immune system has had a chance to catch up. But the antibodies that were produced from that first attack remain in your bloodstream forever; should the invader ever attack again, even years or decades later, these antibodies will come to your defense before you can get sick again.

It's a great system. The only drawback is that you have to get sick once beforeyou can develop that needed immunity for future attacks.

And that's where vaccines come into the picture. Vaccines work by giving your immune system just enough knowledge of a potential invader for your body to produce the antibodies it needs should the real virus ever attack--and the vaccine does so without getting you sick.

Vaccines are made from a weakened or dead virus or the protein that encapsulates the virus. When the vaccine is injected into your system, your immune system reacts by producing antibodies, as if the actual disease were attacking that. These antibodies will give you immunity from any future attacks of the real disease.

Children today routinely receive twice as many vaccinations before kindergarten as they did a decade ago. Like any other medicine, vaccines will occasionally cause reactions, but these are usually mild, perhaps a slight fever or a soreness of the arm where the injection was given. More serious reactions are rare, but they do happen, and, as a consequence, some parents have developed a real fear of vaccines.

One of the reason for these fears lies in the fact that some vaccines made since the 1930s contained thimerosal, a mercury-containing preservative that is used to kill live contaminants in vaccines. At high doses, mercury can cause irreversible nerve and brain damage, but definitely not in the doses found in vaccines. Nevertheless, today none of the vaccines used in the United States to protect preschool children against 12 infectious diseases contain thimerosal as a preservative, with the exception of some flu vaccines.

Protection From the Dirty Dozen

These eight routine immunizations block the 12 diseases children must be vaccinated against.

1. MMR Vaccine

Disease: Measles, mumps, rubella (german measles)

Age for shots and boosters: 12 months and ages 4-6 years.

2. DTaP Vaccine

Disease: Diptheria, Tetanus (lockjaw), Pertussis (whooping cough)

Age: two months, four months, six to 18 months, 4-6 years.

3. Polio Vaccine

Disease: Polio

Age: two months, four months, six months, 12-15 months

4. Hib Vaccine

Disease: Haemophilus influenza type b (Hib disease)

Ages: two months, four months, six months, 12-15 months

5. Hepatitis B Vaccine

Disease: Hepatisis B

Ages: Birth, one-two months, six to 18 months.

6. Varicella Vaccine

Disease: Varicella (chickenpox)

Ages: 12 to 24 months

7. Hepatitis A Vaccine

Disease: Hepatitis A

Ages: 12 months, 18 months

8. Pneumococcal Vaccine

Disease: Pneumococcal disease

Ages: two months, four months, six months, 12 to 15 months

Shoo Flu!

Vaccinating children annually against flu is also very important.

The flu is caused by the influenza virus, which spreads from person to person through coughs and sneezes. For most people, the flu lasts a week or so with symptoms that include sore throat, fever, cough, chills and muscle aches. But some people are at high risk for complications from influenza, which can include convulsions, pneumonia, bronchitis and respiratory infection.

Children age six to 59 months are at high risk for these complications because their immune systems are not fully developed. Doctors recommend that children get their flu shots by October or earlier--before the flu seasons begin.

Click here to check out Dr. Manny's book The Check List (Harper Collins, 2007).

Dr. Manny Alvarez is the managing editor of health news at FOXNews.com, and is a regular medical contributor on the FOX News Channel. He is chairman of the Department of Obstetrics and Gynecology and Reproductive Science at Hackensack University Medical Center in New Jersey. Additionally, Alvarez is Adjunct Professor of Obstetrics and Gynecology at New York University School of Medicine in New York City.

Dr. Manny Alvarez serves as Fox News Channel's senior managing health editor. He also serves as chairman of the department of obstetrics/gynecology and reproductive science at Hackensack University Medical Center in New Jersey. Click here for more information on Dr. Manny's work with Hackensack University Medical Center. Visit AskDrManny.com for more.