When your dog or cat is exposed to a virus or bacteria (pathogens), your pet can become ill in many cases. A good example is canine parvovirus, also called parvo; it’s a highly contagious viral disease that can strike dogs. A puppy infected with parvovirus can have severe vomiting and diarrhea, and even succumb to it in a severe case.
Whether a pup recovers depends on many factors, like nutritional status, parasite load, how much virus it was exposed to, and how long the virus had been attacking the body until treatment was initiated.
The immune system is what will save a puppy. It recognizes the parvo virus as “foreign,” and mounts an attack with antibodies. Once these antibodies have eliminated the virus, and the puppy recovers, the immune system is on the alert for their return at any time in the future. This is called the anamnestic response. If the dog is exposed to the virus some time in the future, the antibodies will overwhelm it early in the process and your pet will not become ill. Your pet now has immunity to that disease.
Vaccinations show the body’s immune system what to protect itself against by introducing a small version of a virus or bacteria that has been rendered harmless. A normally functioning immune system detects this harmless pathogen and formulates antibodies against it. Through the same anamnestic response, the immune system remembers this pathogen for the future and is on the alert for it to appear again.
When vaccinated for a disease, the antibodies produced are on the alert before your pet gets ill. If the immune system is ever exposed to this pathogen again, it mounts an immediate and overwhelming attack on the pathogen, and your pet does not get the disease normally caused by this pathogen. These vaccines are so effective that you do not even know this battle is going on in your pet’s body, which is the way we want it.
An important point in vaccination theory is called community (sometimes called herd) immunity. If enough individuals are vaccinated, the virus cannot travel very fast or well, thus lowering infection rates and sometimes even eliminating the disease in a population altogether.
A good example of community immunity is rabies, a disease fatal to humans and pets. There have been so many dogs that have been vaccinated with the rabies vaccine over the decades that it is very rare to see the disease in a domestic dog—we have never seen a case at our hospital in over 50 years. This is a testimonial to the effectiveness and importance of vaccines. But be aware that keeping this herd immunity to rabies going is important because wild animals such as bats and skunks are reservoir for rabies.