The purpose of vaccinations is to prevent disease and combat viruses. Prior to vaccines, pets frequently died from viral infections. Since the advent of vaccines, death due to viruses that have a vaccine available has significantly declined.
When a normal, healthy kitten is born, the kitten’s immune system has not been exposed to foreign substances or viruses. Through the mother's milk, the kitten will gain some immunity from those viruses the mother is protected against. In the first five to six weeks of life, the mother's antibodies are sufficient to keep the kitten immune from most common viruses. At about five to six weeks, this immunity begins to weaken.
Before the days of effective vaccines, cats routinely died from panleukopenia ("feline distemper") and complications of upper respiratory (herpesvirus, calicivirus) infections. Vaccines are now available to protect against feline leukemia virus infection, feline infectious peritonitis virus and other infections (chlamydia, feline bordetella, ringworm). Current vaccination programs also protect our cats (and us) from the threat of rabies.
Most vaccines are administered by injection (shots), but some newer vaccines can be administered through the nostrils and have been developed to protect against a variety of infections. Some veterinarians believe that annual revaccination is an important and critical part of preventative health care.