When a virus enters the body the body starts producing an immune response. The body makes antibodies that work to stop the virus from being able to use the persons body cells to reproduce.

If the body has not encountered the virus before it will not recognise the unique spikes on the outside of the virus. It will take time for the body to make antibodies that fit the spikes shape perfectly - like jigsaw puzzle pieces. While the body is learning to do this the virus has time to reproduce in host cells and make the person sick.

After some time the body produces antibodies that attach to the spikes and make the virus unable to attach onto any more host cells. The person will start getting better because no new cells are being infected by the viruses.

After an infection the body has some “memory” cells that are able to quickly make the antibodies again if the body encounters the virus again. The person does not get sick again because the antibodies are made quickly, before too many viruses are made.

Vaccines are medicines that are made to prevent us from getting a contagious disease. A dead, weakened or altered form of the pathogen is injected into a person. The vaccine does not contain live or fully functioning pathogens so it cannot give the person the disease. Instead it tricks the immune system into making antibodies even though there is no working pathogen present. Then in the future if the person does get infected with the pathogen the memory cells quickly make the right antibodies and stop the pathogen from infected any cells. The person does not get sick. We say the person is immune to that disease.

Antibodies: Made by the immune system to stop pathogens from causing illness and diseases.

Vaccines: Medicines that prevent living things from getting a contagious disease.

Memory cells: Cells in the human that can quickly make antibodies.

Spikes: Structures on the outside of a virus that antibodies attach on to.

Host: The cell in the body that the virus uses to reproduce.

Immune response: The body's immune system fighting off an infection.

Immune: Unable to catch/be infected by a pathogen, e.g. someone that has received the polio vaccine is immune to polio or someone that has had polio is immune to polio and unable to catch it again.