Although human-to-human transmission of the rabies virus is almost unheard of, the Centers for Disease Control (CDC) speculates that there are several undocumented ways that it might occur. It's theorized that sexual contact, for example, could potentially transmit the virus from one person to another. Kissing, the literal act of "swapping spit," could also probably pass on the disease. The CDC takes it a step further, however, and suggests rabies vaccine treatment not only for people who have had intimate contact with rabid individuals, but even for those with whom they have shared silverware, drinks or cigarettes.
Rabies Transmission: A Picky Virus
Despite rabies' ferocity, it's a picky disease. It's found exclusively in mammals, but even with them, it's still pretty selective. For example, rabies is rarely seen in mice, hamsters, rabbits or squirrels. These creatures are perfectly capable of contracting the virus (Pasteur experimented extensively with rabbits while researching his cure), but rarely do [source: Cohn]. This is probably because animals of this size are unlikely to survive the kind of attack that results in rabies. However, not all small mammals are so fortunate; woodchucks, bats and groundhogs are all common vectors for the disease.
Also, there is still some uncertainty of the virus's effects in regards to host size. Rabies typically runs its course over a period of a few months in most animals, yet in humans the virus may lie dormant for months, even years, before making itself known.
Rabies' selection and treatment of hosts aren't the virus's only unusual traits. Although the disease is easily transferred through both saliva and brain matter, the blood, urine and feces of a carrier pose no threat. Since few animals or people are inclined to go out looking for suspiciously acting creatures' brain matter, outside of saliva, the virus is largely non-transferable. However, rabies is a uniquely adaptive disease, and there are other ways that it has been spread.
Perhaps the most frightening potential method of rabies transmission is through the air (aerosol transmission). It's extremely rare; in fact, there is only one documented case of it happening outside of a laboratory environment. It occurred in a cave that is believed to have housed tens of millions of infected bats [source: Merck Vet]. The virus became airborne through the oral and nasal discharges of the rabid animals, infecting several people who entered the cave. But again, this method of transmission is almost unheard of. In fact, the CDC states that it takes "extraordinary circumstances" for aerosol transmission of the rabies virus to occur [source: CDC].
Although it's also extremely rare, rabies has been spread between humans. It's a very unusual occurrence, and usually only happens through a transfer of tissue in hospitals where unwitting rabid organ donors pass on their infections. This has happened several times with cornea transplants, for example [source: CDC].