According to the Centers for Disease Control (CDC), vaccines help the body learn how to defend itself from disease without the dangers of a full-blown infection. The immune response to a vaccine might cause tiredness and discomfort for a day or two, but the resulting protection can last a lifetime.
Infections are unpredictable and can have long-term consequences. Even mild or symptomless infections can be deadly. For example, most people infected with the human papillomavirus (HPV) never show any sign of infection. But for some, the sign appears years later as an aggressive, life-threatening cancer. By then, it’s too late to get vaccinated.
Vaccines work by imitating an infection - the presence of a disease-causing organism in the body - to engage the body’s natural defenses. The active ingredient in all vaccines is an antigen, the name for any substance that causes the immune system to begin producing antibodies.