A contagious disease is spread directly from person to person. When a high percentage of the population is vaccinated, it is difficult for infectious diseases to spread, because there are not many people who can be infected. For example, if someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone, and it will quickly disappear again. This is called ‘herd immunity’ or ‘community immunity’, and it gives protection to vulnerable people such as newborn babies, elderly people and those who are too sick to be vaccinated.
Herd immunity does not protect against all vaccine-preventable diseases. The best example of this is tetanus, which is infectious but not contagious. It is caught from bacteria in the environment, not from other people who have the disease. No matter how many people around you are vaccinated against tetanus, it will not protect you from tetanus.
Herd immunity only works if most people in the population are vaccinated (for example, 19 out of every 20 people need to be vaccinated against measles to protect people who are not vaccinated). If people are not vaccinated, herd immunity is not guaranteed to protect them.
Credit: The History of Vaccines is an award-winning informational, educational website created by The College of Physicians of Philadelphia, one of the oldest medical societies in the United States. A group of prominent Philadelphia physicians, including Declaration of Independence signer Benjamin Rush, established the College in 1787 “to advance the science of medicine and to thereby lessen human misery.”
Vaccines greatly reduce the risk of infection by working with the body’s natural defences to safely develop immunity to disease. They do this by imitating an infection. This type of infection, however, does not cause illness, but it does cause the immune system to generate T-lymphocytes, which in turn produce antibodies. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.
Vaccines prime your immune system against future “attacks” by a particular disease. There are vaccines against both viral and bacterial pathogens (disease-causing agents).
When a pathogen enters your body, your immune system generates antibodies to try to fight it off. Depending on the strength of your immune response and how effectively the antibodies fight off the pathogen, you may or may not get sick.
If you do fall ill, however, some of the antibodies that are created will remain in your body, playing watchdog after you recover. If you’re exposed to the same pathogen in the future, the antibodies will ”recognize” it and fight it off.
Vaccines work because of this function of the immune system. They’re made from a killed, weakened, or partial version of a pathogen. When you get a vaccine, whatever version of the pathogen it contains isn’t strong or plentiful enough to make you sick, but it’s enough for your immune system to produce antibodies against it. As a result, if you’re exposed to the disease again, your immune system will recognize it and be able to fight it off.
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