Viruses are parasitic.
They can only live and reproduce inside the cells of other animals or plants, in scientific terms, they are called obligate survive in one environment (if a germ can survive in more than one environment, it is called facultative); they are intracellular because they must live inside cells; they are molecular parasites because they are the size of molecules and can only live in a parasitic relationship to another species.
They are very tiny, even the largest of them, the pox virus for example, is about one-fifth the size of an E. coli bacterium, and the smaller ones, among which is the polio virus, are one tenth the size of a pox virus. They cannot be seen with a light microscope. You have to use an electron microscope, X rays, or nuclear magnetic resonance in order to see their structure.
It’s hard to say whether viruses are animals or plants, and in some senses they are just barely alive at all. They consist of nucleic acid, either DNA or RNA, and their sex life, such as it is, consists of their genomes directing the synthesis of their components inside the proper host cell. These components are assembled into new viruses that can then infect other cells. There are many different species, classified by their characteristics. Some contain DNA, others RNA. They vary in shape and size. Some have a lipid “envelope” derived from the membrane of the host cell, as in the case of the virus that causes herpes; others, like polio and the papillomaviruses (the ones that cause warts) lack this envelope. The capsid, the protective shell that surrounds the viral genome, varies in size and shape from one virus to another.
More than a thousand years ago, people noticed that once you recovered from certain illnesses, such as smallpox, you could never get them again. Since getting the disease seemed to protect people against getting it again, they began deliberately infecting people with small amounts of material from smallpox pustules, scratching their arms with it to infect them with a mild version of the disease so that they would be protected against the more serious version.
This worked well in most cases, but in some it led to bad skin lesions all over the body, and about 1 percent of people who underwent this procedure (called “variolation”) died from smallpox. This is not the sort of treatment that would today survive review by the Food and Drug Administration, but for many people it was unquestionably a lifesaver.
Then, about 200 years ago, an English physician named William Jenner noticed that milkmaids, who were commonly infected with cowpox, a mild disease that leaves no scars, were immune to smallpox. (Thus their reputation for having lovely complexions unscarred by smallpox.) So Jenner decided to deliberately infect people with cowpox, risking a mild illness in the hope that it would protect them against smallpox, an often fatal disease. Remarkably, it worked, and the custom of vaccination (Latin vacca = cow) was born.
By the 1890s, Louis Pasteur had figured out how to create an “attenuated” or weakened virus that was strong enough to elicit a protective immune response, but not strong enough to cause disease. This was an important step forward in vaccine safety.
Today there are three different kinds of vaccines: live attenuated virus vaccines, inactivated virus vaccines, and acellular vaccines. The measles vaccine is an example of the first kind, it’s a vaccine made from a live attenuated (weakened) form of the disease-causing virus. Live vaccines are extremely effective, usually one or two doses are enough to protect you for life without ever having to get a booster shot. Rubella and mumps vaccines are also live virus. So are the varicella (chicken pox) vaccine and the oral polio vaccine. The second category is the inactivated virus vaccines. The polio vaccine used in the United States is of this type. The vaccines for diphtheria and tetanus are also inactivated vaccines, but for these diseases and others booster vaccines are necessary throughout life.
Another category is the acellular vaccines. The vaccine for pertussis, for example, contains not the bacterium that causes whooping cough, but only purified antigenic components of the pertussis bacterium, that is, the part of the cell that produces immunity.
Finally, there are vaccines like the one for hepatitis B, which is chemically engineered by recombinant DNA techniques. There are now approved vaccines against 21 different diseases (some diseases can be prevented with more than one vaccine). Eleven of these are recommended for all children; the other ten are used only for populations at risk because of age, residence area, medical condition, or risk behaviors. The reasons why only 11 of the 21 vaccines are recommended for everyone are complex and quite interesting, and we’ll come back to them later when we discuss the problem of the vaccine for meningococcal meningitis.
The most important thing to do to prevent infectious disease in children is to get them vaccinated, do it on time, and keep the vaccinations current. There are now safe and effective childhood vaccines that will prevent diphtheria, Haemophilus influenzae Type B (Hib), hepatitis A, hepatitis B, measles, mumps, pertussis (whooping cough), polio, rubella (German measles), tetanus (lockjaw), and varicella (chicken pox). These diseases are now very uncommon in the United States, thanks to an effective vaccination system. But they flourish elsewhere in the world, in places only a plane ride away, and that makes efficient vaccination essential even in a place where these diseases have been largely eliminated.
Unlike some of the things we’ve discussed (like sanitizing your laundry or disinfecting your toothbrush, for example), getting these vaccines, and making sure that your kids get them, is unquestionably essential for your continuing good health. There are some fears about side effects of various vaccines, but the latest studies show that this risk is dwarfed by the dangers of getting any of the vaccine-preventable diseases.
Often the study that shows a connection between a given childhood illness and a vaccine gets much more publicity than the numerous studies that contradict those findings. Widely publicized assertions that the MMR vaccine is associated with increased risk for autism (based on a small study that included 12 children) have been roundly refuted, but many parents are aware only of the original unscientific assertion, not the contradicting studies.
The Centers for Disease Control conducts ongoing studies of vaccine safety in a project called the Vaccine Safety Datalink (VSD). VSD has data on more than 6 million people, and their medical records are examined continuously for adverse events relating to vaccination. At any given time, the CDC is conducting dozens of studies of various vaccines and publishing the results of their findings. In other words, these recommendations are not made casually or carelessly. They are based on the latest scientific knowledge and are modified as new knowledge is acquired.
In addition, the CDC runs a program called the Vaccine Adverse Event Reporting System (VAERS), and anyone can report what he or she believes to be an adverse reaction to a vaccine. Most reports to VAERS come from manufacturers of vaccines and health care professionals, but about 3 percent come from patients or parents of patients. The VAERS web site is at http://vaers.hhs.gov