On June 11, the WHO signaled that a pandemic of H1N1 flu had begun. The virus, sometimes called swine flu, has spread from Mexico to all corners of the world, but it has also been less deadly that initially feared.

Still, a leading health expert told reporters in Washington Tuesday, the illness should not be underestimated.

“As far as the severity of the disease, so far we’ve been lucky,” Dan Rutz, a former CNN health reporter who has conducted seminars around the world on swine flu, said. “It seems that this pandemic flu has not caused the severe illness for most people. Now having said that, it is still a dangerous disease, and it causes severe illness in some people, especially pregnant women and people who have other underlying health problems, such as asthma, heart disease or diabetes.”

The virus was first detected in the US in April and likely spreads in much the same way seasonal influenza spreads. However, Rutz said, it may be more fatal.

“Almost everybody in the population would be susceptible to the pandemic flu, whereas only some people would be susceptible to the seasonal flu in any given year,” he said.

Meanwhile, vaccines remain the most powerful health tool for control of any influenza, and the US government is working closely with manufacturers to take steps to manufacture a H1N1 vaccine.

“The seasonal flu vaccine will not protect against swine flu, but you need both this year,” Rutz said. “They work separately, but your body can handle it. The seasonal flu vaccine will protect you against circulating strains of seasonal flu virus; the swine flu vaccine will protect you specifically against this one virus.”

The US expects more of the influenza to appear in coming months, as the seasons change from summer to autumn and winter. Rutz said preventative measures were still important, such as hand-washing, covering the mouth during coughs and sneezes and staying apart from those who are already ill.