Swine flu has now killed up to 149 people in Mexico, there are twice as many confirmed cases in the U.S. today as yesterday, and two more countries - Canada and Spain - have reported their first confirmed swine flu cases.
A phase 4 alert indicates a significantly increased risk of a pandemic, a global outbreak of a serious disease.
About one million people died in 1968 during the last such outbreak, the “Hong Kong” flu pandemic.
The World Health Organisation pandemic scale ranges from phase 1 (low risk of a flu pandemic) to phase 6 (a full-blown pandemic is under way). A phase 4 level indicates person-to-person transmission in a limited geographical area; phase 5 indicates that a virus is present in several places so an imminent upgrade is likely.
To date however, there have been no deaths reported outside Mexico, although the reported cases in the US have doubled from 20 yesterday to 40 today.
The United States, Britain, France and Germany have all urged their citizens to avoid non-essential travel to Mexico and other areas affected by swine flu.
There have been six lab-confirmed cases of mild swine flu in Canada and one in Spain, which became the first country in Europe to confirm a case after a man who returned from a trip to Mexico last week was found to have the virus.
Spain has 26 suspected cases under observation and a New Zealand teacher and a dozen students who recently travelled to Mexico are being treated as likely mild cases.
Countries including Australia, France, Germany, Norway, Sweden, Israel, Guatemala, Costa Rica and South Korea are all testing suspected cases of the flu.
In the first confirmed cases in Britain, Scotland's health minister says two people tested positive for swine flu.
The flu has been mild everywhere except Mexico and the reasons for that aren't clear. Most of those who died were between 20 and 50 years of age which has been a hallmark of past pandemics.
Salon magazine reports that while you should take swine flu seriously there’s no need to panic yet.
At this stage symptoms of swine flu are similar to normal flu: fever, chills, aches, coughing and congestion. What’s different is some vomiting and diarrhoea.
The best way to avoid it is to “wash your hands and cover your mouth with the crook of your elbow if you cough or sneeze. If you get sick, yes, call your doctor, but treat yourself with rest, fever reducers (not aspirin for children) and plenty of fluids. We always have to worry about the very young and old, but again, we haven't seen swine flu in these groups yet”, reports the magazine.
Swine flu is not caught from eating pig meat products, but that hasn’t stopped several countries whacking import bans on pork from the US, while airline and leisure stocks are taking a hit as investors worry about the impact on travel.
However, shares in makers of drugs and vaccines such as Roche are higher.