Diagrams of the general models used to describe the transition dynamics of MRSA in nursing homes. (A) the model with three states which assumes that a resident may be MRSA free (Negative), colonized with USA300 and, possibly, co-colonized with non-USA300 (USA300) or colonized with non-USA300 (non-USA300); (B) two-state model which considers colonization with any MRSA strain or a specific strain, either non-USA300 or USA300. In the strain-independent model, Positive represents colonized with any MRSA strain and Negative stands for non-colonized with MRSA. In the non-USA300 model, Positive represents colonized with non-USA300 while Negative represents free of non-USA300, irrespective of USA300. Similarly, in the USA300 model, Positive stands for colonized with USA300 and Negative represents free of USA300, irrespective of non-USA300 strain-type.