Immunization (also called vaccination) is one of the greatest achievements in public health. Before vaccines, many adults and children died from diseases like measles, whooping cough (pertussis), and the flu. Vaccines can prevent these illnesses and many other diseases.
When you get a vaccine, it helps to prevent you from getting the targeted disease. If enough people get vaccinated, the disease does not have much chance to spread. This is called Herd Immunity, which means the entire community is better protected. Polio is a great example of a crippling disease that has been eradicated because of the high percentage of polio immunization.
Getting vaccinated against serious illness is a healthy behavior and highly recommended.