Immunizations are a way of boosting the body's immune system with specific immunity to protect the individual being immunized. They also provide “herd immunity” and reduce the overall disease burden in the community
The body has two kinds of immunity: general and specific. General immunity is related to being fit and healthy and having good nutrition. Specific immunity means having an immune system that recognizes and can fight specific diseases.
There are usually only two ways a person can acquire specific immunity to a particular disease; by catching it or by immunization. Catching the disease causes the immune memory cells to store the antigens particular to that disease. If the same disease agent is encountered again, the memory cells can trigger a specific immune response much more quickly the second time around, with the result that the person often will not experience the illness at all. This is the reason why people don't usually catch measles or chickenpox twice.
Immunization gives the body an encounter with the antigens found in the disease without having to catch the “wild” form. Immunizations can be made with killed or with “live attenuated” viruses/bacteria. Both forms of immunization allow the body to create memory cells which can trigger the specific immune response on encountering the disease agent.
Even though immunizations are not 100% effective, as they depend on the body's immune responses, they can often reduce the severity of the disease and the complications experienced as a result of the disease. For example the measles virus - even though people who have been immunized against measles can occasionally catch the disease, it is much rarer for an immunized person to develop the complications of measles such as meningitis or SSPE (sub-acute sclerosing pan-encaphalitis) which can be fatal.
Immunizations are also important for the community as a whole, through the effect known as herd immunity. This means that when the majority of the community is immune to a particular disease, there is less and less of the disease circulating and therefore less chance of catching it. This benefits everyone, especially those who are too young to be immunized or whose immunity has worn off or who are immunocompromised. If enough of the community can be immunized, a disease which is only found in humans can be made completely extinct, as has already happened with the smallpox virus. Polio is the next target for worldwide extinction. Research is currently being conducted into malaria and AIDS immunizations.
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