Viruses can have beneficial effects  For example, some kill more-complex microorganisms, some of which are deadly to humans  But viruses have such simple structures that replacing just a few of a beneficial virus' s several million atoms can make it deadly to humans
Clearly, since alterations of greater complexity than this are commonly produced by random mutations, any virus could easily become dangerous to humans
Some microorganisms that are more complex than viruses are beneficial to humans.