The more exposure the flu virus has to the drugs - and in some countries it is available as a preventive treatment before people even catch the infection - the more chance it has of working out how to evade their effects.

BBC: Flu drug 'shows promise' in overcoming resistance

While the conventional wisdom is that treatment for disease should be undertaken whenever possible (bear in mind that in the early days of AIDS treatment, not everyone supported treatment among the poor), it is clear that some of the best results come from preventive tactics.

FORBES: An Ounce of Prevention