Hi,
Yes, unfortunately new viruses are popping up all the time -- I recall seeing somewhere a figure on how many new ones per day, but the number escapes me right now.
Thank the avast team for doing a great job of keeping up to date with the definitions updates -- two a week is downright quiet for us.
There's been times when two or even three new sets of definitions have been issued the same day, if there's a flood of new viruses in-the-wild.
Best,
Mike