The viruses are stopped by isolation and effective infection-control methods, therefore if these measures are put in place quickly, they can control an infection and potentially reduce its spread.

It is impossible to wipe out everyone at the same time therefore there would be warnings if there were to be a deadly virus and people could prepare for it. Everyone has a different immune system and therefore some can fight off an infection better than others, therefore not everyone would be wiped out or as badly affected.

The UK is one of the most prepared countries when it comes to an influenza pandemic. They have a detailed contingency plan for dealing with an outbreak when or if one occurs. These are laid out by the Department of Health http://www.dh.gov.uk/en/Publicationsandstatistics/Publications /PublicationsPolicyAndGuidance/DH_4121751

This policy includes measures to reduce the spread of an influenza pandemic through the rapid production of vaccines and stockpiling anti viral drugs.