Vaccines are prepared from harmful viruses or bacteria and administered to patients to provide immunity to specific diseases. The various types of vaccines are classified according to the method by which they are derived.

The most basic class of vaccines actually contains disease-causing microorganisms that have been killed with a solution containing formaldehyde. In this type of vaccine, the microorganisms are dead and therefore cannot cause disease; however, the antigens found in and on the microorganisms can still stimulate the formation of antibodies. Examples of this type of vaccine are the ones that fight influenza, typhoid fever, and cholera.

A second type of vaccine contains the toxins produced by the microorganisms rather than the microorganisms themselves. This type of vaccine is prepared when the microorganism itself does little damage but the toxin within the microorganism is extremely harmful. For example, the bacteria that cause diphtheria can thrive in the throat without much harm, but when toxins are released from the bacteria, muscles can become paralyzed and death can ensue.

A final type of vaccine contains living microorganisms that have been rendered harmless. With this type of vaccine, a large number of antigen molecules are produced and the immunity that results is generally longer lasting than the immunity from other types of vaccines. The Sabin oral antipolio vaccine and the BCG vaccine against tuberculosis are examples of this type of vaccine.

3.Which of the following expresses the main idea of the passage?

4. How many types of vaccines are presented in the passage?