In fact, there is a clear historical record of infections having been acquired in the laboratory. More than 4,000 laboratory-acquired infections have been reported and many others have occurred. Incidents such as a splash to the face, needle puncture, contamination of open wounds or skin lesions, ingestion or inhalation of an aerosolized infectious agent and other incidents have all produced laboratory-acquired infections. These are designed either for work with or without experimental animals. The principles of infection need to be kept in mind when appropriate biosafety precautions are selected.
Agents vary in their virulence, or capacity to infect and cause disease.
Whether disease results from infection is dependent upon the inoculum or the number of infectious agent particles presented to the host, the route of transmission and the innate capabilities of the agent. As an example, purified Mycobacterium tuberculosis is classified in RG 3 because of the serious consequences of infection and the large number of viral particles present. BSL2 precautions are designed to protect against breaks in the skin, such as those from punctures or open skin lesions by providing barriers. BSL3 precautions are designed primarily to protect against aerosols. Although the bloodborne pathogens are not normally transmitted by aerosols of blood or body fluids, BSL3 precautions may be required when working with concentrated cultures or when performing procedures involving aerosolization of blood or body fluids containing small amounts of virus.