Many types of vaccines involve the injection of dead parts or germ cells into people or animals to provoke a small immune response. In theory and in practice, this form of vaccination is very successful, and when people or animals come into contact with live germ cells, their bodies are already immune to them. Another form of vaccination is the live vaccine. This is injecting living parts or all germ cells into a person/animal body to create immunity as well.
Most types of live vaccines, and there are many, are also called live attenuated vaccines. This attenuation process is extremely important because it makes it possible to inject live germs into people to gain immunity. Exposure to live germ cells would usually end up making people sick with the virus or bacteria. With attenuation, this risk is dramatically reduced for healthy populations.
Essentially, attenuation is a process by which germs or infectious parts of viruses or bacteria are reduced. This could be done in a number of ways, including putting a virus into an egg, often a chicken, that contains an embryo, or infecting animals with a virus because they can fight and change it. In this process, the virus or bacteria changes, so it is very unlikely, but not impossible, to infect someone who receives a vaccine with the new form of virus, but the vaccination will still confer immunity to people who contract it. The live vaccine is still live virus, but it has changed to a less viral and more beneficial form.
There are several vaccines that are offered as a live vaccine. Oral polio has used live poliovirus for many decades. Recently there has been a shift to more people receiving inactivated or killed poliovirus, due to concerns about possible contracting the disease. It was always a risk when the vaccine was developed, but the risk tended to be much less than the potential risk associated with contracting polio from a wild polio source. Infection with a live attenuated virus is generally milder than infection from contact with wild virus.
Other forms of live vaccine include the nasal spray used as an alternative to the flu shot. This is an attenuated virus as well. Some additional examples include the varicella or chickenpox vaccine, measles, mumps, rubella (MMR), and vaccines against some forms of tuberculosis.
While most people can handle live vaccines, some people are told not to use them. People who have a weakened immune system are generally not recommended to get the life-attenuated vaccine. These pose a higher risk of contracting the disease.
Live vaccines potentially carry the risk of causing infection among perfectly healthy people as well. People concerned about this problem should talk to their doctors about the statistical chance of infection from the vaccine compared to the statistical chance of getting extremely sick without immunity. Another thing people should know before getting a live vaccine is if they are allergic to eggs. Attenuation processes often use eggs, and people can have allergic reactions to injections with certain live vaccines if they have previous egg allergies.