This animation shows how a virus takes over a cell and then sends millions of its children away to spread infection through a system.
In order to reduce the risk of being infected, your immune system needs to be on top form to destroy the virus before spreading to healthy cells. Most of the time, your immune system has got things under control, but when more people around you are getting sick, your immune system needs to be extra vigilant to check all the viruses floating around.
When your immune system encounters viruses, they have to first recognise them as intruders before they can do anything about them. In order for your body to do this, it has to create the B and T cells specially designed to target the virus.
Essentially, this is how a vaccine works. Your body is exposed to a dead or weakened version of a virus, and the immune system recognises it as bad, so it creates the T and B cells to combat it. You don't actually get sick from this process, you are just teaching your body which cells it needs to target. Without a vaccination, your body has to basically work a little harder in order to recognise and combat all the viral infections at the same time, before the viruses can cause an illness.