"We are not sure of human to human transmission, as last time the experts from the centre for disease control and prevention (CDC) of Atlanta, USA did not find any evidence of the virus spreading though man-to-man transmission," said Professor Mizanur Rahman, director general of Health Services.

"Detail studies are required and two experts on community and hospital infection controls from the CDC are here to evaluate the samples. Only after they carry out the studies, we can be sure of the mode of its transmission," Prof Rahman said.

Prof Abul Kalam Azad, director of Institute of Epidemilogy, Disease Control & Research (IEDCR), said, "We are worried about the spread, but are now concentrating on massive awareness-raising programme in the areas where people are becoming sick."

Local members of parliament, imams, doctors and teachers have been involved in the awareness-raising programme to stop spread of the virus.

He, too, said they were yet to find out the actual mode of transmission of the virus and that the scientists from the CDC would confirm it after investigations in Atlanta.

Prof Azad said, since April 16 last, no new case had been reported in the areas, where several expert medical teams were working to find the cause of the disease.

The CDC experts would start investigations, mainly sample collection, in the areas of Mallikpur, Guhalaxmipur, C&B Ghat, Ali Char and Kadirpur areas in Faridpur from today.

Meanwhile, the health directorate says a team of experts with a portable device, which instantly identifies virus and its transmission mode, is expected here soon from Health Canada, a specialised centre like the CDC.

"We hope, when Health Canada is here, we will be able to do more close investigations much more quickly," said Prof Azad.

As of April 20, preliminary reports of one cluster of 30 cases, including 18 deaths, attributed to Nipah virus infection in Faridpur district has been received.

The laboratory testing, done by the CDC, has confirmed Nipah virus infection in 16 of the cases.

Nipah virus is a newly recognised zoonotic virus, that is, a virus that animals pass to humans. The virus was 'discovered' in 1999. It has caused disease in animals and in humans through contact with infectious animals. It is currently believed that certain species of fruit bats are the natural hosts of it.

Though the exact mode of transmission from animal to animal or from animal to human is uncertain, it appears to require close contact with contaminated tissue or body fluids from infected animals.

Nipah antibodies have been detected in pigs and some other domestic and wild animals. According to the World Health Organisation (WHO), the role of species other than pigs in transmitting the infection to other animals has not yet been determined.

The incubation period for the virus is between four and 18 days. In many cases, the infection is mild or non-apparent (sub-clinical). In symptomatic cases, the onset usually comes with 'influenza-like' symptoms, with high fever and muscle pains.

The disease may progress to inflammation of the brain (encephalitis), with drowsiness, disorientation, convulsions and coma. Fifty percent of clinically apparent cases of Nipah virus infection die.