Because of their ability to undergo FREQUENT MUTATIONS, VIRUSES CAN RAPIDLY EVOLVE.
Viruses are one of the microscopic organisms that causes diseases in plants, animals and humans. Diseases cause by virus are usually difficult to treat because of the capacity of viruses to undergo frequent mutations; that is, they change their genetic components in a very rapidly manner, in such a way that, their genetic components are never the same for long. This makes it difficult for scientists to make effective drugs against viruses. Two different viruses can also combine together to form a new and unique type of virus. This is the major reason why diseases such as HIV, do not have a cure yet.