Put simply, vaccines give our bodies a head start in fighting an illness.
If we are naturally exposed to a bacteria or virus, which are also known as pathogens, our immune system will produce antibodies to attack and destroy that particular bacteria or virus.
The immune system will also create antibody-producing memory cells, which will remember the pathogen and will be able to respond immediately should we be exposed to it again.
When we're exposed to a bacteria or virus for the first time it takes time for our immune systems to respond, putting us at risk of becoming ill.
Vaccines contain weakened or inactivated parts of an antigen, which is the part of a bacteria or virus (pathogen) that prompts our immune system to make antibodies.
Newer vaccines contain a small part of the genetic instructions or blueprint of the antigen instead of the antigen itself.
In both cases, the vaccines will not give us the disease. But they will prompt our immune system to react as if we had contracted the actual disease.
Any temporary side effects from a vaccine, such as a fever or aches and pains, are down to the response of the immune system and are usually short lived.
Having a vaccine means we will have antibodies, which are like the soldiers of the immune system, ready and waiting to fight that specific bacteria or virus should we come into contact with it naturally in future.
It's safer to have a vaccine than to contract an illness and wait for our immune system to fight it off.
Most vaccines have been in use for decades, with millions of people across the world receiving them every year.
Like all medicines, vaccines must go through extensive and rigorous testing, including on volunteers, to make sure they are safe before they can be added to a vaccination programme.
No vaccines are 100% effective and effectiveness differs between vaccines.
Some vaccines may stop us from becoming ill in the first place. After having others we may still contract the disease, but the illness is likely to be milder and not last as long.
When a lot of people are vaccinated, illnesses find it hard to circulate. This herd immunity helps to protect those who cannot have vaccines, such as very young babies, or those with underlying health conditions or severe allergies.
Vaccines are one of the most important things we can do to protect our own health and that of our families and communities.
According to the World Health Organization, vaccination currently prevents between three-and-a-half to five million deaths worldwide every year from diseases like diphtheria, tetanus, whooping cough (pertussis), flu and measles.
A worldwide vaccination programme, together with other measures such as prevention programmes, eradicated deadly smallpox in 1980.
Polio, which is caused by a virus and causes paralysis and death mostly in children, has been eradicated in most parts of the world thanks to vaccination. Efforts continue to eradicate it completely.