A biological hazard, or biohazard, is anything coming from living organisms (i.e. pollen, fungi, animals, insects, bacteria and viruses) that could be a threat to someone's health. It is represented by ☣, the biohazard symbol, which is used everywhere in the world. When people see this sign they know to take precautions, and to follow proper conduct for science labs.
There are four biosafety levels made by the Center of Disease Control. These are used in laboratories so only people who have the right skills can enter. Different levels mean scientists must wear different uniforms and take special care when dealing with dangerous things like viruses.
Biosafety Level 4 BSL-4 is for very hazardous diseases that kill many people and are hard to treat. BSL-4 workers must be careful and alert at all times and wear suits with special air filters. A lot of hemorrhagic fevers are dealt with in BSL-4, like Ebola, Marburg, and Lassa Virus.