It was the early 1890s, and Ernest Hankin was studying cholera outbreaks along the banks of the Ganges. As the locals dumped their dead in the holy water, the river should have quickly transformed into a poisonous spring of the disease, with an epidemic sweeping through towns and villages down the valley.

He had seen this across Europe as water supplies became infected with the bacteria, yet here, on the banks of the Ganges, the disease remained relatively tame; the new outbreaks simmered and then died out rather than spreading like wildfire.

Hankin concluded that something mysterious within the water was killing the germs before they could wreak havoc, but it took another 20 years for a French scientist to suggest that their guardian angel was a type of virus known as a bacteriophage. Harmless to humans but deadly to the cholera bacteria, the virus appeared to be purifying the water before it could infect the local bathers.

Long ignored by scientists, it is now thought that these “ninja viruses” may one day save millions of lives, far beyond the banks of the Ganges, as they offer us a new arsenal of weapons against deadly disease.