“FITNESS, defined as the overall capacity of a virus to produce infectious progeny has long been recognized as a critical parameter that reflects virus adaptation to a specific environment. Unperturbed replication of large viral populations in a constant environment results in fitness gain, adaptation, while repeated bottlenecks that result in accumulation of mutations result in fitness decrease, de-adaptation.”
http://www.sciencedirect.com/science/article/pii/S0042682215001580