A category A infectious substance is in a form that is capable of causing permanent disability or life-threatening or fatal disease in otherwise healthy humans or animals when exposure to it occurs. Exposure would occur if the substance were released from its protective packaging and a human or animal came into contact with it.

Some examples of category A infectious substances include:

• Bacillus anthracis (cultures only)
• Brucella abortus (cultures only)
• Brucella melitensis (cultures only)
• Burkholderia mallei (cultures only)
• Clostridium botulinum (cultures only)
• Dengue virus (cultures only)
• Escherichia coli, verotoxigenic (cultures only)
• Ebola virus
• Francisella tularensis (cultures only)
• Hantaviruses causing hemorrhagic fever with renal syndrome
• Herpes B virus (cultures only)
• Human immunodeficiency virus (cultures only)
• Lassa virus
• Mycobacterium tuberculosis (cultures only)
• Poliovirus (cultures only)
• Rabies and other lyssaviruses (culture only)
• Shigella dysenteriae type I (cultures only)
• West Nile virus (cultures only)
• Yersinia pestis (cultures only)

This is not an exhaustive list. Sometimes, deciding on the classification of an infectious substance requires professional judgement and involves knowing the medical history or symptoms of the source patient or animal and/or knowing the local epidemiological conditions at the time the patient specimen or culture was obtained.

If there is doubt as to whether or not a substance meets the criteria of category A, it must be treated as a category A substance for shipping.