Smallpox is a disease caused by a specific virus, while the common cold can be caused by over 100 different viruses. explain why it is possible to develop a vaccine to prevent smallpox, but it is difficult to develop a vaccine to prevent the common cold. in your answer be sure to: • identify the substance in a vaccine that makes the vaccine effective • explain the relationship between a vaccine and white blood cell activity • explain why the response of the immune system to a vaccine is specific • state one reason why it would be difficult to develop a vaccine to be used against the common cold?