Viruses and bacteria are formidable foes, and your body’s immune system can’t always win the war against them on its own. Fortunately, scientists have learned how the body’s immune system actually works – and how we can further arm it with vaccines.
Vaccines are variants or derivatives of pathogenic microbes that cause the immune system to mount defenses to the pathogen itself. They’ve been so successful in boosting the body’s defensive powers that they’ve virtually wiped out one of the greatest scourges humans have ever known: smallpox.
In the late 18th century, English physician Edward Jenner became fascinated by a milkmaid’s assertion that she wouldn’t get smallpox because she had been exposed to a milder version of the disease: cowpox (the word “vaccine” comes from vacca, the Latin word for cow). So, in 1796, Jenner scratched an 8-year-old boy with a needle containing fluid from the sore of a milkmaid with cowpox. When exposed to smallpox, the boy did not come down with the disease.
The exposure to cowpox had caused the boy’s immune system to form antibodies. Later, when faced with smallpox, his immune system “remembered” the pathogen and mounted a faster, more intense immune response.
It’s no exaggeration to say that the smallpox vaccine is one of humanity’s greatest achievements. In the 20th century alone, smallpox killed more than 300 million people, three times as many as were killed in all the wars of that century combined. Prior to that, the disease killed untold millions, slaughtering kings and commoners alike and changing the course of history again and again.
For example, the Spanish conquistadors’ greatest weapon against the indigenous people of Mexico wasn’t the musket or the cannon – it was the smallpox virus they carried. Exposed to a disease their immune system had never seen, the Aztecs died in droves. Today, the smallpox virus exists only in Russian and American labs. The last natural case occurred in 1977, on the heels of a worldwide immunization drive.
Most vaccines still work the same way as the one that eradicated smallpox. They use live but greatly weakened versions of pathogens to give you a mild infection akin to the serious infection you really don’t want to get. Your immune system’s response to the mild infection arms it with the antibody ammunition it needs to seek out and destroy the more serious infection later.
Sometimes it doesn’t work – the vaccine doesn’t stimulate enough of a response to do a body good. Very, very rarely, it backfires – the live pathogen in the vaccine triggers big trouble by itself. But “live attenuated vaccines” remain the only medical defense there is for many diseases, especially viral ones, since viruses are immune to antibiotics.
Other kinds of vaccines try to limit exposure to live pathogens by using pathogens that have been killed by chemicals, or by using pieces of pathogens – just the fragments needed to elicit your immune system’s response. Because these vaccines don’t contain live invaders, they don’t cause even minor infection. Yet they’re also less effective in stimulating your body’s antibody production and tend to require booster shots.
Still other vaccines, called toxoids, don’t use live pathogens, dead pathogens, or even pathogen pieces. They use toxins from bacteria. For example, a tetanus shot contains toxins secreted by the bacteria that cause tetanus. Once this poison is scientifically stripped of its toxic power, and becomes a toxoid, it can help your body build the antibodies it needs to neutralize terrible tetanus toxins in you.
Victories vs. Vexations
Scientists continue to develop new vaccines. Chickenpox, caused by the varicella-zoster virus, used to be a rite of passage for children in United States. Yet since 1995, there’s been a vaccine against the highly contagious disease. Made from a live attenuated virus, the vaccine prevents chickenpox in 90 percent of patients. The few kids that do get chickenpox now tend to have mild cases.
Still, viruses and bacteria evolve and mutate as fast as scientists discover ways to thwart them, if not faster. HIV, the virus that causes AIDS, has infected more than 60 million people since scientists first began noticing its devastating effects. In some countries, more than 30 percent of the adult population now carries the virus. Other deadly diseases, such as Ebola, are evolving, too. The war continues.