Doctors treating the first surge of the flu in WA this year warn that the dominant strain so far is one of the most contagious and dangerous.
Despite its relatively late start, the flu season could be brief but intense because the most common virus type is H1N1, the influenza A strain responsible for the 2009 pandemic and known to be a leading cause of hospitalisation and death in young adults and the middle-aged.
Though this year's flu vaccine covers H1N1, as well as influenza B and A/H3 which are also circulating, many young and middle-aged people do not have the annual jab.
The WA Health Department confirmed that the number of flu cases was rising, as were visits to GPs.
The Eastern States, which tended to get flu activity slightly ahead of WA, were having more cases than at the same time last year.
Communicable disease control medical co-ordinator Paul Effler said it signalled the flu season was likely to hit WA in the next few weeks.
"These warning signs are a reminder for people who want to be protected that they should get the vaccine sooner rather than later, as it can take up to two weeks for the body to build up good immunity," Dr Effler said.
"The protection from the vaccine lasts about a year, so even if you were vaccinated last year you'll need to have it again to remain protected."
Australian Medical Association WA past president Richard Choong said that over the past few weeks GPs had started seeing a stream of patients with the flu and other respiratory illnesses.
"We're fortunate that the season has started late, but it's a concern that the type we're mostly seeing is H1N1, which is quite contagious, so it's very important that if people get sick, they stay home from work or school," Dr Choong said.
Dr Effler said there was still time to get the flu vaccine because the season was yet to peak.
The flu could cause serious illness, particularly in the elderly, pregnant women and people with underlying medical conditions.
Parents were also encouraged to get their young children immunised.