Overview of Viruses By Laura D Kramer, PhD, Wadsworth Center, New York State Department of Health Reviewed / Revised Mar 2023 View Patient Education Diagnosis Treatment Prevention Viruses are among the smallest microbes, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer in length (megavirus, pandoravirus) have recently been discovered.