The onset was a single monkey coming down with pneumonia at the California National Primate Research Center in Davis. In a matter of weeks, 19 monkeys were dead and three human beings were sick.

A new report confirms that the Davis outbreak was the first known case of an adenovirus jumping from monkeys to humans. The upside: the virus may one day be harnessed as a tool for gene therapy, said a report in sciencemag.

Adenoviruses are responsible for a variety of illnesses in cattle, dogs, horses, pigs, and other animals, but scientists thought the viruses and their ailments couldn't jump between species.

But, on 14 May 2009, a healthy adult male titi monkey, a small, reddish-brown species that calls much of South America home-came down with a cough at the Davis primate center and soon became lethargic and wouldn't eat.

Staff members tried to save the animal by giving it intravenous fluids and antibiotics, but its condition worsened, and after 5 days staff members euthanized him. Just after four weeks, another titi monkey showed the same symptoms and it went on and on with other monkeys.

It just took 2 months for 23 of the 65-strong population to become sick, and 19 eventually died. A team led by infectious diseases researcher Charles Chiu of the University of California, San Francisco, analyzed lung tissue samples from the dead monkeys and identified a never-before-seen adenovirus, which they named titi monkey adenovirus (TMAdV), said the Sciencemag report.

Where did the virus originate?

It's unlikely it originated in the titi monkeys themselves, Chiu says: Hosts that are that susceptible to a disease are not likely to be its originators.

The researchers later found out that one person, who had had close daily contact with the monkeys, reported coming down with a fever, chills, a headache, a dry cough, and a burning sensation in her lungs-hallmarks of the kinds of respiratory infection commonly caused by adenoviruses. Two of her family members reported similar, though less severe, symptoms in subsequent weeks, the report said.

The lab worker and her family members recovered from the condition within four weeks, without seeking any medical attention. But, it was too late for researchers to swab for traces of the adenoviruses directly. Thus they examined the patients' blood for antibodies and compared them with those found in the infected monkeys. The lab worker and one of her family members showed a match, suggesting that the monkeys gave the virus to the lab worker or vice versa. But, when the team tested a representative set of 81 blood samples from donors in the western United States, none had antibodies. That suggests humans weren't the source of the outbreak either.

However, after testing the other monkeys at the primate center, the researchers found one healthy rhesus macaque with TMAdV antibodies. That suggests the disease might have arisen in the macaques and somehow passed to lab workers or the titi monkeys via shared medical equipment or some other contact between the two species, the researchers reported in PLoS Pathogens.

Although the virus so far has not proven deadly, or even all that serious, to the humans, the new findings suggest there may be more pathogens than previously thought with species-jumping potential. Now we need to broaden our focus in looking at monkeys' and other animals' adenoviruses, Chiu says. We've only touched the tip of the iceberg.