An infection happens when a "microorganism" (a living thing so small that we can't see them individually) begins to grow and multiply in the body.
We all have millions of microorganisms on and in our bodies. There are many bacteria (microorganisms that can live independently) on our skin, and even more in our intestines and mouths. These bacteria are not only fairly harmless (unless your immune system isn't working quite right) but actually help you out. You must have bacteria in your intestines to help you digest food: to take one example, Vitamin K, which is necessary for your blood to clot properly when you have a cut, can be manufactured by certain bacteria in your intestines. For some people (those who don't like broccoli and other green vegetables) the bacteria are the major, if not the only, source of Vitamin K.
However, there are many bacteria that will make you sick. Some are usually not found in the body unless you are sick. In other cases, normally harmless or helpful bacteria may overgrow or invade parts of the body where they aren't supposed to be. There are also viruses, particles that cannot live by themselves but which can invade your body's cells, take over those cells' growth machinery, and make the cells start producing more virus particles. (Computer viruses are called "viruses" because their behavior in computers is very similar to the way human and animal viruses behave.)
Our bodies are vulnerable to infections from many bacteria and viruses. Because of that we have many natural defenses, collectively called the "immune system", designed to fight infections. Some infections are harder for us to fight off than others, but for many of the serious viral diseases (for which there are no antibiotics) and some bacterial infections it is possible to induce immunity with a vaccine made from components of the infecting bug or the toxin (biochemical poisons) that some bacteria produce, which will prevent future infections with the natural, full-strength bug. Many viruses in particular cannot live in anything other than the human body, and immunization can actually eradicate viruses. Smallpox, which as recently as the 1950's was still a major killer in many parts of the world, now exists only in freezers in two laboratories -- one in the United States and one in Russia -- and even these lab samples may soon be destroyed, leaving the world permanently safe from smallpox.
The United States Centers for Disease Control and Prevention (CDC) has just finished and published a study of how vaccines have affected the number of patients infected with vaccine-preventable infections over the last few decades (back to 1900, in the case of smallpox): of the ten diseases they looked at for which we have had vaccines for more than 25 years, eight (diphtheria, measles, mumps, polio (both acute and paralytic; the two forms were counted separately), rubella (German measles, including congenital infection which was also counted separately), and smallpox) did not kill anyone in the United States in 2004 (the last year for which they had complete information on reported causes of death) and three of these diseases (diphtheria, polio, and smallpox) were not seen at all in the US in 2006 (the last year for which they had full reports of cases of infections). Compare this with the number of deaths reported at peak for some of these diseases: 7518 for whooping cough in 1938, as opposed to 27 in 2006, 2720 for acute polio in 1949 and 3145 for paralytic polio in 1952, compared with none from either form of polio in 2006, or 3065 for diphtheria in 1936 and none in 2006. The article was published in the Journal of the American Medical Association (volume 298, page 2155; issue of November 14, 2007).
Although some of the available vaccines can give you immunity after a single dose no matter when you receive it, others have to be given several times at certain ages and intervals to provide full protection. With some diseases your immunity will eventually wear off, and so you will need booster immunizations at intervals to stay fully protected. Because of this, it's important to keep your child's (and your own) immunization schedule up to date.
It is also possible to harvest antibodies from people who are immune to certain diseases, purify them (the purification does not necessarily remove antibodies to other diseases, but it does remove other impurities from the mixture -- including viruses such as HIV), and give them to a patient who is not immune to that particular disease. The antibodies will help the recipient fight off infections. The protective effect of these immune globulins (also know as gamma globulins) will last only as long as the antibodies last, since they can't be replenished constantly by the body. Therefore immune globulin is not a permanent substitute for vaccination. However, in some cases the immune globulin will help you fight off an infection until your body starts making its own antibodies.
Here I have described many of the common diseases we immunize patients against, and talk about the vaccines we use as well. The schedule for each vaccine is also given; the schedule changes occasionally as we discover more about diseases and the protective effects of vaccines, but your doctor will know the latest recommended schedule and will help you follow it. I also talk about combination vaccines, which combine vaccines against several different diseases in a single shot and cut way down on the number of pokes a child needs while still giving full protection.
It's always a good idea to keep a copy of your child's immunization record at home, too. Besides being useful if schools or day-care centers need the information (as they usually will at some point) it can also come in handy if your child is ill and the doctors need to make sure that s/he is not susceptible to a particular bug. Most states in the United States now have computer-based immunization registries for children which any health-care provider can check, but even so you should have an up-to-date record yourself.
Some of the available flu vaccines contain thimerosal, a preservative used in many vaccines and other medicines (including contact-lens storage and cleaning solutions) to prevent bacteria from growing in the solutions. Until a few years ago, many vaccines contained thimerosal, but it has been removed from almost all of the vaccines we now give to children. The only widely-used vaccine that still contains thimerosal is the influenza vaccine, and only some manufacturers still produce flu vaccine that contains thimerosal.
Thimerosal is a mercury-based chemical. There is a theoretical risk of mercury poisoning from the thimerosal used to preserve vaccines. Although several researchers claim that there is evidence of such poisoning, and in particular an increased risk of autism with thimerosal-containing vaccines, this has not been well-proven, and several large studies in different countries show no change in the occurence of autism or other neurodevelopmental disorders between children given thimerosal-containing and thimerosal-free vaccines. A new study by California public-health authorities (published in the January, 2008 issue of the Archives of General Psychiatry and quoted in the New York Times) found that the autism rate in children rose continuously from 1995 to 2007 even though thimerosal was removed from all vaccines, except for some flu vaccines, in 2001. If thimerosal was in fact responsible for autism, autism rates should have fallen after 2004. They didn't.
Influenza infection is dangerous to infants. However, only two of the three flu vaccines on the market are meant to be given to children under age 2 years, and those vaccines are not recommended for children younger than six months. Therefore, most public health authorities, including the Centers for Disease Control and Prevention, recommend that infants at risk receive flu vaccine starting at age 6 months whether or not the vaccine contains thimerosal. Some of the current vaccine is made with a reduced amount of thimerosal, reducing the theoretical risk of mercury poisoning, and that vaccine should be used for vaccinating children under age 3 years. The other vaccines we routinely give to children are all available without preservative.
I am aware that there are many people who believe that thimerosal should not be used to preserve vaccines. I believe that thimerosal should not be used to preserve vaccines if at all possible. I also know several people with autism and Asperger's syndrome, including one in my own family. However, I also believe that vaccine-preventable diseases are sufficiently dangerous to patients, especially the very young (many of whom I admit to the hospital every year with complications of flu, whooping cough, and other vaccine-preventable diseases), that any risk from the preservative is MUCH less than that from the diseases themselves, and I would not hesitate to vaccinate my family against those diseases even if I could not obtain preservative-free vaccine. (That is my opinion. Other people's opinions differ. As always, you need to talk to your own or your child'd doctor to help decide if you or your child should receive vaccines.)