BEIJING – The ease with which the H7N9 avian flu virus passes from birds to humans may make the new bird flu one of the most lethal viruses to emerge so far, according to officials with the World Health Organization.

China’s National Health and Family Planning Commission invited a team of experts from WHO to visit affected areas around Shanghai and Beijing in order to provide recommendations on prevention and control of the virus. WHO officials said the virus doesn't appear to be easily spread human-to-human. But public health authorities are concerned because the virus seems to spread through poultry flocks causing no visible signs of illness in the birds. So farmers or anyone handling live poultry may not be aware that their birds are infected.

Speaking to reporters at a media briefing, Dr. Keiji Fukuda said the outbreak is an evolving situation, and he suggested that only the most serious infections are being reported while a large number of infections are going unreported, according to the Associated Press.

To date, China has recorded 21 deaths and 104 laboratory confirmed cases of the virus.