A biological hazard, also known as a biohazard, is an organism or a by-product from an organism that is harmful or potentially harmful to other living things, primarily human beings. Common types of biological hazards include viruses, medical waste, or toxins that were created by a particular organism or microorganism. The "biohazard symbol" is a familiar sight in hospitals, and any object that carries it should be treated with extreme caution.

There are four levels of biohazards, classified by the Center for Disease Control and Prevention (CDC) in the United States. A level 1 biological hazard poses the least risk while a level 4 poses the greatest. The CDC has a great number of procedures in place for the prevention of a biological hazard disaster, and many different ways to handle and clean up small and large-scale contaminant disasters.

Level 1 biological hazards consist mainly of bacteria and other microorganisms, which pose little risk in the case of exposure and can be warded off through the simple use of gloves or a mask. They can generally be disposed of in their own separate trash container without worry and are easily decontaminated. Level 2 biohazards consist of viruses and bacteria that can have a limited detrimental effect on humans, for example they may cause a disease such as salmonella poisoning, hepatitis, measles, Lyme disease and more. People working in the presence of these biological hazards will usually exercise a substantial amount of care in their handling and disposal with proper hand, eye and body protection.

When biological hazards are considered to be at level 3, they become much more serious, since contact with them can now be fatal if left untreated. Examples if biohazards at this dangerous level are anthrax, West Nile Virus, malaria, typhus and more. People that aid victims of biological hazards at this level must be well trained and use very specific safety equipment and clothing so they do not contract a deadly disease. The same is obviously true of level 4 biohazards, for which there are no known treatments, are generally fatal, and are easily spread through contact and through the air. Someone working in the vicinity of level 4 biohazards like the Ebola virus or dengue fever, for example, must use an airtight Hazmat suit with his or her own oxygen supply.

A universal symbol for biohazards that was created in 1966 by Dow Chemical is used to signify the presence of hazardous biological materials. The biological hazard symbol can be used in many places such as hospitals, laboratories, chemical companies and other places where there is a threat of hazardous contamination. You might see the symbol in a doctor’s office or a place where hypodermic needles are collected, such as a blood and plasma donation center. A hazard that is toxic but not a biohazard is denoted simply by a symbol of a skull and crossbones.