Cats that survive a natural infection usually develop sufficient, active immunity to protect them for the rest of their lives. Mild cases may go unnoticed and also produce immunity. It is also possible for kittens to receive immunity from their mother through the transfer of antibody through the blood when in utero or through the milk while nursing. This passive immunity from the mother is temporary and its effectiveness varies in proportion to the level of antibody in the mother's body. The immunity diminishes rapidly and is not considered effective after 8 weeks of age.
Vaccines offer the safest protection.
Most vaccines are made from live viruses treated to destroy their ability to cause disease. They stimulate the cat's body to produce protective antibodies against the virus to prevent infection by natural, disease causing viruses. The vaccines are very effective but are preventative, not curative. They must be administered before the cat is exposed and infected to be effective. Most young kittens receive their first vaccination between 6 and 8 weeks of age, or as soon as they are taken from their mother.