Infectious substances are materials known to contain, or suspected to contain, a pathogen that has the potential to cause disease in humans and animals. These materials are exempt from normal placarding procedures, so it can be very difficult to determine the hazard before one has been exposed to the material. A pathogen is a micro-organism such as a bacteria or a virus.

Examples include blood and body fluids, sharps, and cultures of infectious materials such as anthrax or influenza virus.