30. The common cold is a viral disease. So is AIDS, which is caused by the Human Immunode- ficiency Virus, or HIV. Effective vaccines against these viral diseases have not been developed despite years of
research and work by dedicated scientists. One reason for this is the rapid muta- tion rate of these viruses,
leading to new surface proteins in a very short time. Return to Model 2 to speculate about how this rapid
change in the cold virus and HIV make vaccinations difficult to develop.