A biohazard refers to any biological substance that poses a threat to the health of living organisms, primarily humans. This includes bacteria, viruses, medical waste, and biological toxins that can cause disease or harm. Biohazards are classified into levels based on the risk they present, from mild (Level 1) to extreme (Level 4). Common settings dealing with biohazards include hospitals, laboratories, and disaster sites. Proper handling, containment, and disposal of biohazardous materials are critical to preventing the spread of infection and protecting public health.