Virology is the branch of microbiology that deals with viruses and viral infections. The overall goal of virus research is understand the action of various viruses and develop vaccines or techniques that are effective at preventing or treating the diseases caused by them. Viruses that affect humans range in severity from the rhinoviruses that cause the "common cold" to the human immune deficiency (HIV) virus that causes AIDS.By their very nature, viruses are highly contagious and therefore affect millions of people, plants, and animals. This field continues to have new discoveries that are important to researchers and clinicians in the field.