Bats are the most common source of rabies in humans in North and South America, Western Europe, and Australia. In the United States, there were 19 cases of human rabies from 1997–2006, 17 of which were attributed to bats.[27] In North America, about half of human rabies instances are cryptic, meaning that the patient has no known bite history.[24] While it has been speculated that rabies virus could be transmitted through aerosols, studies of the rabies virus have concluded that this is only feasible in limited conditions. These conditions include a very large colony of bats in a hot and humid cave with poor ventilation. While two human deaths in 1956 and 1959 had been tentatively attributed to aerosolization of the rabies virus after entering a cave with bats, "investigations of the 2 reported human cases revealed that both infections could be explained by means other than aerosol transmission".[28] It is instead generally thought that most instances of cryptic rabies are the result of an unknown bat bite.[24] Bites from a bat can be so small that they are not visible without magnification equipment, for example. Outside of bites, rabies virus exposure can also occur if infected fluids come in contact with a mucous membrane or a break in the skin. Rabies virus has also been transmitted when an infected human unknowingly dies of rabies, and their organs are transplanted to others.[28]