<em>Vaccines work by simulating an illness to help establish immunity. However, while this sort of infection nearly never produces sickness, it does trigger the immune system to create T-lymphocytes and antibodies. The imitation illness can sometimes induce modest symptoms, like as fever, after receiving a vaccination.</em>
Explanation: Vaccines help develop immunity by imitating an infection. This type of infection, however, almost never causes illness, but it does cause the immune system to produce T-lymphocytes and antibodies. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever.
Highway signs help reduce the rate of car accidents, help ensure the safety of pedestrians, and help drivers know how to communicate with other drivers in a non-verbal way that keeps us all safe.