1. Mutation is any alteration in a gene from its natural state. This change may be disease causing or a benign, normal variant. Specific mutations and evolution in influenza viruses cannot be predicted, making it difficult if not impossible to know if or when a virus such as H1N1 might acquire the properties needed to spread easily among humans.
2. Process of adaptation for viruses. A virus reproduces by infecting a cell. Viruses change their surface proteins so they can attach to cell surface receptors. "The influenza virus genome has remarkable plasticity because of a high mutation rate and its segmentation into eight separate RNA molecules. This segmentation allows frequent genetic exchange by segment reassortment in hosts co-infected with two different influenza viruses. Source: http://www.avianinfluenza.org.
(2008, Geary W.Skick. Protecting your business in a pandemic : plans, tools, and advice for maintaining business continuity. ISBN: 978-0-313-34602-6)