* Poor in developing countries at heightened risk, Chan says
* Pregnant women, chronic patients are most vulnerable
* If virus shed in faeces, could spread due to bad sewage
GENEVA, May 18 (Reuters) - The new H1N1 flu poses a particular threat to chronically ill people and pregnant women, and experts fear it could take on new momentum if it starts to spread through human waste or to combine with other viruses, the head of the World Health Organisation said on Monday.
"The world of today is more vulnerable to the adverse effects of an influenza pandemic than it was in 1968, when the last pandemic of the previous century began," WHO Director-General Margaret Chan said.
Most patients with underlying chronic conditions such as heart disease or diabetes -- already shown to be most vulnerable to the new H1N1 virus -- live in the developing world, Chan said in her main speech to the WHO's annual assembly of ministers.
The global burden of chronic diseases has shifted dramatically from rich countries to poor ones in recent years, she said. About 85 percent of all cases and deaths from such ailments are concentrated in low-and middle-income countries.
"The implications are obvious. The developing world has, by far, the largest pool of people at heightened risk for severe and fatal H1N1 infections," Chan declared.
The new virus -- a genetic mixture of swine, bird and human viruses -- has officially infected 8,829 people in 40 countries, killing 74 people, according to the United Nations agency.
Separately, the bird flu virus H5N1 is firmly entrenched in poultry in several countries, mainly in southeast Asia. "No one can say how this avian virus will behave when pressured by large numbers of people infected with the new H1N1 virus," Chan said.
If the new H1N1 swine flu virus is shown to be shed in faeces, it would introduce an additional route of transmission, especially in developing countries, according to the WHO chief.
People living in crowded, squalid urban shantytowns with poor sanitation systems could face additional risk, she said.
Both the cholera and polio viruses, which have killed and maimed in developing countries, are spread through weak sanitation and sewage systems.
Millions of people already depend on life-extending drugs to survive HIV/AIDS and resurgent forms of tuberculosis, Chan said. "Most of these people live in countries where health systems are already overburdened, understaffed and poorly funded," she said.
Chan said pregnant women seemed to be at heightened risk of severe or fatal H1N1 infections, posing the threat the new strain would increase already unacceptable levels of maternal mortality, which are closely linked to weak health systems.
The situation was expected to worsen as the financial crisis drives more people to forego private health care and turn to publicly-financed systems, according to Chan, a former director of health in Hong Kong.
"What will happen if sudden surges in the number of people requiring care for influenza push fragile health systems over the brink?," she asked. (Reporting by Stephanie Nebehay; Editing by Jonathan Lynn and Janet McBride)