There are two major subtypes for Influenza A virus--H1N1 pandemic and H3N2.
There are two major lineages for Influenza B strain--B/Victoria-like and B/Yamagata-like.
Typically, influenza A virus strains are responsible for the most illnesses each year.
He said this year, the B-Victoria strain of flu, which is seen typically at the end of the season, is already here.
"Influenza B viruses tend to be disproportionally more severe in children and young adults," said Piedra.
He said each flu season is different. This year, it's starting earlier.
"We are seeing both Influenza A H1N1 Pandemic and Influenza B-Victoria. Influenza B viruses are circulating early into the year, when we normally see them in a later part of the year," said Piedra.
He said the flu season in the Southern Hemisphere, which is during our summer, was not a severe one, so health experts didn't predict it for the Northern Hemisphere.
Piedra said It's not too late to get vaccinated. Also, anti-viral drugs are available for people with high-risk conditions, as well as those who have break-through influenza infections after being vaccinated.
He added mutations happen and predicting which flu strain will hit, is not a perfect science or art.