A global influenza pandemic is under way, according to Dr. Margaret Chan, director-general of the World Health Organization.
"We are in the earliest days of the pandemic," she said in a statement today. "The virus is spreading under a close and careful watch."
"Globally, we have good reason to believe that this pandemic, at least in its early days, will be of moderate severity," she said. "As we know from experience, severity can vary, depending on many factors, from one country to another."
The overwhelming majority of patients experience mild symptoms and make a rapid and full recovery, often without any medical treatment, she said.
Worldwide, the number of deaths is small and officials don't "expect to see a sudden and dramatic jump in the number of severe or fatal infections," she said.
In almost all areas with large and sustained outbreaks, most cases have been in people under 25 years old, she said.
Most cases of severe and fatal infections have been in people between 30 and 50 years old, she said.
"This pattern is significantly different from that seen during epidemics of seasonal influenza, when most deaths occur in frail elderly people," she said.
Many severe cases have been in people with underlying chronic conditions, such as respiratory diseases, notably asthma, cardiovascular disease, diabetes, autoimmune disorders and obesity, she said.
"At the same time, it is important to note that around one third to half of the severe and fatal infections are occurring in previously healthy young and middle-aged people," she said.
And "without question, pregnant women are at increased risk of complications,"' she said. "This heightened risk takes on added importance for a virus, like this one, that preferentially infects younger age groups."
"Finally, and perhaps of greatest concern, we do not know how this virus will behave under conditions typically found in the developing world," she said. "To date, the vast majority of cases have been detected and investigated in comparatively well-off countries."