How severe will the flu outbreak be?Epidemiologists race to pin numbers on the global H1N1 spread.Declan ButlerMobile health units check people with symptoms.O. TORRES/AFP/Getty ImagesThe World Health Organization (WHO) this week remained on the verge of declaring a pandemic of the H1N1 swine-associated flu virus. Public-health bodies and scientists have made progress in starting to understand the outbreak, but major questions remain about how severe the disease will get.As Nature went to press,