The biosafety level is an agreed protocol level of precautions required to be implemented in an containment facility to isolate potentially harmful biological agents (e.g. virus, bacteria, fungi, prions, toxins etc). The biosafety levels are 1, 2, 3, 4. 4 being the highest and most dangerous level.
The levels were devised by the Centers For Disease Control And Prevention (CDC) in the USA. These biosafety levels have been adopted by the EU.