By Nicola Fell
BBC News correspondent
Doña Porcela uses her own potions as a treatment
Rain is pelting down on Doña Porcela's treatment room in Puerto Cabezas, the main town on Nicaragua's Northern Caribbean coast.
The room is barren except for a few plastic chairs, a wooden table and some old plastic bottles balanced precariously on timber beams.
Doña Porcela is a respected traditional healer here and the bottles are filled with her secret medicinal potions.
Her patient today is a teenage girl asleep on a piece of cardboard, serving as a mattress on the dirt floor.
"Grisi Siknis turns people into witches and they go crazy," she said.
Last year there were 65 cases of Grisi Siknis, which translates from the local Miskito language as 'crazy sickness'.
It behaves like a virus, sending teenager after teenager into a frenzied state followed by long periods of coma-like unconsciousness.
Lola Emberto's children had attacks
While Western doctors are at a loss as to how to treat this mysterious illness, Doña Porcela says she can cure people with her concoction.
"It can be drunk or bathed in," she said. "Within three or four days, they are normal again."
Using her herbs and candles, she performs a cleansing ceremony on sufferers and, often, on their houses too - akin to an exorcism.
She travels anywhere anxious parents ask her to come, such as Lola Emberto's home, set among palms trees and red earth in Barrio Pobre, a poor suburb of Puerto Cabezas.
"For four months, my children were affected," said Mrs Emberto.
"I couldn't sleep or eat. My daughter was just running around like a maniac. She tore off all her clothes.
"One time, she fell into the well while suffering an attack.
"Other times, she'd run into the bush or into the river and people would try to catch her when they could."
Both her 18-year-old son and her 13-year-old daughter were affected.
Mrs Emberto, like many Miskito people, believes in black magic and that Doña Porcela lifted a curse to cure her children.
Lola lives in the poor suburb of Barrio Pobre
The first recorded cases of this mass hysteria date back to the 1850s.
Until recently, it only affected Miskito people but 16-year-old Anadina Smith is of Spanish descent.
"One day at school, I felt giddy, and found it difficult to breathe," she said.
"Then, I saw something coming towards me - a kind of black man or a dragon that entered me and possessed me."
Three other girls were affected the same day at Anadina's school, where Reverend Harold Dixon is headmaster.
He has witnessed many attacks of Grisi Siknis.
"They get giddy and they faint and fall to the ground," he said.
"Then, they start hollering and they hit their heads on the wall or desk. They have an extra strength.
"You have to have five or six people to hold down one girl."
On the outskirts of Puerto Cabezas, at Uraccan University, Professor Pablo McDavis has been researching Grisi Siknis for the last few years, in the Indigenous Diseases Department.
"We have taken samples of blood from patients while suffering an attack and, in a lab, we can't detect anything," explains a puzzled McDavis.
"Drugs or injections tend to only increase a patient's aggressiveness. Clinically we can't detect anything.
"It is like an outbreak. If an attack is not contained quickly, it can spread throughout an entire community."
So far this year there have been 46 cases of Grisi Siknis, and, as it continues to confound Western medical classification, business for Doña Porcela remains brisk.