If you are exposed to the same infection again, these memory cells recognise it and trigger a new immune response against it immediately. This usually prevents the infection from causing illness again. They stimulate the body to produce memory cells, so the immune system is 'primed' to fend off an infection as soon as it enters the body for the first time. In some cases, extra 'doses' or boosters may be needed, or it may prove impossible to generate protection. Types of vaccine Immunisations are given in the form of vaccines, which are usually either injected into the body or swallowed.
Inactivated vaccines contain the whole organism after it has been killed.
These are non-infectious and cannot cause illness, but the immune system still recognises them and produces memory cells. Live attenuated vaccines contain the whole organism after it has been weakened but not killed. These are the best vaccines for stimulating protective immunity, but can cause a mild attack of the disease they are designed to protect against. Sub-unit vaccines and inactivated vaccines are all safe for people with HIV, as are some live attenuated vaccines. However, you may be warned not to receive certain live attenuated vaccines, especially if your CD4 count is low.