A biological hazard or biohazard is an organism, or substance derived from an organism, that poses a threat to (primarily) human health. This can include medical waste or samples of a microorganism, virus or toxin (from a biological source) that can impact human health. It can also include substances harmful to animals. The term and its associated symbol is generally used as a warning, so that those potentially exposed to the substances will know to take precautions. There is also a biohazard HCS/Workplace Hazardous Materials Information System (WHMIS) logo which utilizes the same symbol.

In Unicode, the biohazard sign is U+2623 (☣).

Biohazardous agents are classified for transportation by UN number:

• UN 2814 (Infectious Substance, Affecting Humans)
• UN 2900 (Infectious Substance, Affecting Animals)
• UN 3373 (Diagnostic Specimen or Clinical Specimen or Biological Substance, Category B)
• UN 3291 (Medical Waste)

Levels of biohazard

main: Biosafety level The United States' Centers for Disease Control and Prevention (CDC) categorizes various diseases in levels of biohazard, Level 1 being minimum risk and Level 4 being extreme risk.

• Biohazard Level 1: Bacteria and viruses including Bacillus subtilis, canine hepatitis, Escherichia coli, varicella (chicken pox), as well as some cell cultures and non-infectious bacteria. At this level precautions against the biohazardous materials in question are minimal, most likely involving gloves and some sort of facial protection. Usually, contaminated materials are left in open (but separately indicated) trash receptacles. Decontamination procedures for this level are similar in most respects to modern precautions against everyday viruses (i.e.: washing one's hands with anti-bacterial soap, washing all exposed surfaces of the lab with disinfectants, etc). In a lab environment, all materials used for cell and/or bacteria cultures are decontaminated via autoclave.

• Biohazard Level 2: Bacteria and viruses that cause only mild disease to humans, or are difficult to contract via aerosol in a lab setting, such as hepatitis A, B, and C, influenza A, Lyme disease, salmonella, mumps, measles, scrapie, and HIV. "Routine diagnostic work with clinical specimens can be done safely at Biosafety Level 2, using Biosafety Level 2 practices and procedures. Research work (including co-cultivation, virus replication studies, or manipulations involving concentrated virus) can be done in a BSL-2 facility, using BSL-3 practices and procedures. Virus production activities, including virus concentrations, require a BSL-3 facility and use of BSL-3 practices and procedures", see Recommended Biosafety Levels for Infectious Agents.