Hendra virus is a potentially fatal zoonotic virus that is transmitted from horses to humans. Together with the closely related Nipah virus, a cause of respiratory disease and encephalitis in pigs and humans, these two viruses form the genus Henipavirus in the family paramyxyoviridae.

First described following an outbreak of acute, fulminant (sudden, severe, and intense) respiratory disease in horses in 1994 in a north Brisbane, Australia, suburb, hendra virus has since been found to naturally infect a variety of Pteropid bat species native to Australia. Since 1994 there have been 26 reported incidents of hendra infection affecting 63 horses, one dog, and seven people. Of these 26 incidents, 12 have occurred in 2011. No reports of bat-to-human transmission of this virus have been made. All seven cases of hendra in humans have, to date, been associated with equine infection. The case-fatality rate of human hendra virus infection is 57%. Horses testing positive for hendra virus are euthanized, according to national guidelines.

Although the host bat species are native to large regions of coastal Australia, equine and human cases of hendra virus infection have been reported only in the north of New South Wales and in Queensland. Serological evidence of hendra infection has been demonstrated in all four species of fructivorous (fruit-eating) bats found on the Australian mainland; this evidence of seroconversion is not geographically restricted to the areas where equine cases have been reported. Bats with antibodies to hendra have been identified from Melbourne in the south to the far north of Queensland. Seroprevalence increases with age.

Hendra virus infection of horses is a very problematic disease for equine clinicians. Cases present with a variety of clinical signs, which poses serious human safety concerns. The first large outbreak of hendra virus infection in 1994 was associated with severe respiratory distress and sudden death that occurred in the majority of affected horses within 36 hours of the onset of clinical signs. hendra virus has a definite tropism (a biological phenomenon, indicating growth or turning movement of a biological organism) for the vascular endothelium (organ lining), particularly the arterial endothelium, which explains many of the post-mortem findings, such as foam-filled airways, dilated pulmonary lymphatics, severe pulmonary edema, and congestion