On August 18 and 19, 2006, thousands of people thronged Mumbai's Mahim beach as word spread that the sea water had miraculously turned sweet. Many thought it was a blessing from Makhdoom Ali Mahimi, a 13th century Sufi saint whose shrine is located nearby. Despite official warnings that the water was unsafe, people were seen bathing in it to 'cure their ailments', and filling bottles with the 'holy water' to drink. The episode resulted in a massive traffic jam that lasted several hours. As Charles Mackay observed in his 1841 classic, Extraordinary Popular Delusions and the Madness of Crowds, "Men, it has been well said, think in herds; it will be seen that they go mad in herds while they only recover their senses slowly, and one by one."
There was no miracle in Mahim beach. The inflow of fresh water from swollen rivers and an upsurge of groundwater after heavy rains tend to reduce the salinity of seawater. The Amazon releases so much freshwater into the Atlantic Ocean in the rainy season that potable water can be drawn from the ocean up to several miles.
What Mumbai witnessed was a 'collective delusion', defined as the rapid spread of false but plausible beliefs. It can be positive and take the form of wish-fulfilment. But usually it is negative and spreads through fear. Collective delusions differ from superstitions in that they occur in an unorganised, spontaneous fashion and are usually temporary.
The episode of the 'Monkey Man' who was reportedly clawing, biting and harassing the people of Delhi in May 2001 is a famous case. A much-publicised incident in 1938 in the US involved Orson Welles' live radio enactment of HG Wells' novel, The War of the Worlds, featuring an invasion of Earth by Martians. It was so credible that about 1.2 million people rushed into the streets, made hysterical phone calls to authorities, prayed in churches and scrambled madly for buses and trains.
Mass hysteria can also have a humorous side, like the koro epidemic, in which men are convinced that they are the victims of a contagious disease causing their penises to shrink and retract into the body. In Singapore in 1967, thousands of anxious men rushed to hospitals clamping their penises between chopsticks lest they retracted. Similar episodes occurred in North-east India in 1982 and in Hainan Island, China, in 1984-85. Thankfully, each time, the only result was a lot of bruised egos and private parts.