Anti-virus software developers can't predict what happens in the future and when a virus problem is unforeseen it sometimes cannot be detected until such time as the virus updates are issued.
So you must keep your virus software up to date at all times.
Generally virus problems arise from people opening documents or clicking on things and panicking when they see messages encouraging them to open files that they shouldn’t open!
If you are vigilant and careful then you can avoid a lot of common virus attacks. But if you are unfortunate to suffer an attack, we are here to help straightaway!