With fears about bird flu cases in humans on the rise, the World Health Organisation has warned that new infections and re-emergence of the old might be a cause of worry for the scientific community in this century.

"We should be prepared for the worst. Surveillance should be increased in every country," Dr N Kumara Rai, director (Communicable Diseases) for the South East Asia Region, told PTI in Delhi.

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The simultaneous occurrence of highly pathogenic avian influenza or bird flu in several countries is 'historically unprecedented and of great concern'. While last year the world was gripped by Severe Acute Respiratory Syndrome (SASR), this year it is bird flu, an infection affecting birds but which has reportedly been transmitted to humans in some cases.

"The problem with such diseases is that these spread rapidly and have a high mortality as the human body is not immunised against them," Rai said.

Prof Anoop Misra of the Department of Medicine at AIIMS said, "That not many humans have been infected so far is an encouraging sign but care needs to be taken as influenza is known to cause pandemics. Besides, it is not known if Indians will show the same symptoms as we had only a mild infection during the SARS outbreak."

While SARS was a new disease, bird flu is not with the first epidemic being reported in Italy more than 100 years back. However, only in recent times have humans reported being affected.

The spread of bird flu to humans in the current outbreak has been confirmed in Thailand, Vietnam and China, Rai said, adding WHO was also investigating possibility of human-to-human transmission of infection, though it has not been reported so far.

There are 15 sub-types of avian influenza virus and all can be transmitted to humans. What causes an infection to spread to humans is not known, Rai said.

The current outbreak in most countries is due to the H5 N1 virus though a different strain has been reported in Pakistan. India reported the H9 type last year, which was also found in the Netherlands where it claimed the life of one human and showed up in 83 others.