Biological hazard

# Overview
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, 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/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 3733 (Diagnostic Specimen or Clinical Specimen or Biological Substance, Category B)
- UN 3291 (Medical Waste)

# Levels of biohazard
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: Several kinds of bacteria 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: Various 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, HIV, scrapie.
- Biohazard Level 3: Various bacteria and viruses that can cause severe to fatal disease in humans, but for which vaccines or other treatment exist, such as anthrax, West Nile virus, Venezuelan equine encephalitis, SARS, smallpox, tuberculosis, typhus, Rift Valley fever, Rocky Mountain spotted fever, yellow fever.
- Biohazard Level 4: Exclusively viruses that cause severe to fatal disease in humans, and for which vaccines or other treatments are not available, such as Bolivian and Argentine hemorrhagic fevers, dengue fever, Marburg virus, Ebola virus, hantaviruses, Lassa fever, Crimean-Congo hemorrhagic fever, and other various hemorrhagic diseases. When dealing with biological hazards at this level the use of a Hazmat suit and a self-contained oxygen supply is mandatory. The entrance and exit of a Level Four biolab will contain multiple showers, a vacuum room, an ultraviolet light room, and other safety precautions designed to destroy all traces of the biohazard. Multiple airlocks are employed and are electronically secured to prevent both doors opening at the same time. All air and water service going to and coming from a Biosafety Level 4 lab will undergo similar decontamination procedures to eliminate the possibility of an accidental release.