Difference: Hematologist; Virologist; Immunologist; Epidemiologist Hematologist is a doctor whose field of expertise encompasses all diseases and disorders related to blood. virologist may be both a physician who diagnoses, manages and prevent infections and also a scientist seeking new knowledge on the viruses that cause infections. Immunologist studies how the body protects itself from microbes that cause infectious disease and tumors. Also develops new vaccines, novel antibiotics and improved therapies for inflammatory diseases. Epidemiologist studies the distribution and determinants of health-related states and events (not just diseases) in specified populations. Infectious disease physician diagnoses and manages the bacterial, viral, fungal, and parasitic infections that occur in humans, using existing knowledge.