Special pathogen: A highly infectious agent that produces severe disease in humans. Special pathogens include the viruses that cause several hemorrhagic fevers, such as Ebola hemorrhagic fever, Lassa fever, and hantavirus pulmonary syndrome, and other recently identified and emerging viral diseases, such as Nipah virus encephalitis. These viruses are RNA-coded (often negative-stranded or ambisense in coding strategy), encased in a lipid envelope, and display a degree of aerosol infectivity in the laboratory. In addition, all of the viruses are vector-borne zoonotic agents (meaning that under normal conditions, these viruses exist in animals); the majority are found in rodents, but some occur in other mammals or arthropods as well. All of these viruses are classified as Biosafety Level 4 (BSL-4) pathogens and as such must be handled in special facilities designed to contain them safely. The coronavirus that causes SARS (severe acute respiratory syndrome) is considered a special pathogen.