Herd immunity is an important infectious disease concept. Basically, it involves trying to ensure that a high enough percentage of a population is resistant to an infectious disease so that the disease cannot be spread easily through the group. Ensuring that a large percentage of the population is vaccinated helps protect individuals that cannot be vaccinated (because of allergy, disease or other reasons) or that did not properly respond to vaccination (not all vaccines protect all vaccinated individuals).
“Herd immunity” usually refers to this concept when applied to herds of animals, but the “herd” can be a small local population, a regional population, or broader, and it can be people or animals. For some human infectious diseases, it’s been shown that vaccination of 75-95% of the population is required to prevent outbreaks. If vaccination rates start to slip, the chance of an outbreak increases. This is best seen in some areas where vaccination rates decline in certain groups of kids because parents are reluctant to have their children vaccinated (for one reason or another), and subsequently outbreaks of disease start occurring (or increasing).
Vaccination is an important (but not the only!) infection control tool. For diseases that are transmissible between dogs (or cats, people, or whatever other “herd” is being considered), vaccination of a single animal helps protect that individual from disease, and also helps protect the rest of the population.
During a public health infectious disease course that I teach, a student showed us a link to this interesting and amusing demonstration of herd immunity from the UK. Make sure you have your sound turned on. It’s an entertaining description of the concept of herd immunity.