Human Body Systems and Vaccines

Which of the following could replace 'contagious', as used in the passage?

Directions: Read the following passage carefully and select the most suitable option according to the corresponding number in the passage.

Vaccines
A vaccine is a biological preparation that improves immunity to a particular disease. Vaccines were first used by Edward Jenner (England) in the 1770s to inoculate small pox using the cow microbe. Vaccines have resulted in the eradication of small pox, one of the most contagious and deadly diseases known to man.
A vaccine is usually made from a weakened or attenuated form of the microbe that it is intended to fight. When the body recognizes the virulent microbe attack, it neutralises the target microbe before it can enter body cells.