Viruses regularly evolve new ways of making people sick, but scientists usually do not become aware of these new strategies until years or centuries after they have evolved. In a new study published on Thursday in the journal Science, however, a team of scientists at Michigan State University describes how viruses evolved a new way of infecting cells in little more than two weeks.

The report is being published in the midst of a controversy over a deadly bird flu virus that researchers manipulated to spread from mammal to mammal. Some critics have questioned whether such a change could have happened on its own. The new research suggests that new traits based on multiple mutations can indeed occur with frightening speed.

The Michigan researchers studied a virus known as lambda. It is harmless to humans, infecting only the gut bacterium Escherichia coli. Justin Meyer, a graduate student in the biology laboratory of Richard Lenski, wondered whether lambda might be able to evolve an entirely new way of getting into its host.

The standard way for lambda to get into a cell is to latch onto its outer membrane, attaching to a particular kind of molecule on the surface of E. coli. It can then inject its genes and proteins into the microbe.