On the eve of Onam, the most joyous festival in India's Kerala state, 45-year-old Sobhana lay shivering in the back of an ambulance, drifting into unconsciousness as her family rushed her to a medical college hospital.

Just days earlier, the Dalit (formerly known as untouchables) woman, who earned her living bottling fruit juices in a village in Malappuram district, had complained of nothing more alarming than dizziness and high blood pressure. Doctors prescribed pills and sent her home. But her condition spiralled with terrifying speed: uneasiness gave way to fever, fever to violent shivers, and on 5 September - the main day of the festival - Sobhana was dead.

The culprit was Naegleria fowleri - commonly known as the brain-eating amoeba - an infection usually contracted through the nose in freshwater and so rare that most doctors never encounter a case in their entire careers. "We were powerless to stop it. We learnt about the disease only after Sobhana's death," says Ajitha Kathiradath, a cousin of the victim and a prominent social worker.

In Kerala this year, more than 70 people have been diagnosed and 19 have died from the brain-eating amoeba. Patients have ranged from a three-month-old to a 92-year-old man.

Normally feeding on bacteria in warm freshwater, this single-cell organism causes a near-fatal brain infection, known as primary amoebic meningoencephalitis (PAM). It enters through the nose during swimming and rapidly destroys brain tissue.

Kerala began detecting cases in 2016, just one or two a year, and until recently nearly all were fatal. A new study has found only........

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