How does mrsa go from a local infection to a disseminated one?

MRSA: Disseminated MRSA infections are the infections caused by strains of staph aureus that are resistant to the antibiotic methicillin and are not restricted to the original site of infection but have spread to different parts of the body. Hospital-acquired disseminated MRSA infections have occurred frequently in individuals with reduced immunity. However, community-acquired infections are increasing.

Immune breakdown: Your body is very good at confining staph infections to a localized area, unless the infection is so severe that it just overwhelms the body's defenses, or the immune system breaks down. When either of those things happens, the infection can spread, either through the blood or the lymphatic channels, to other parts of the body. Mrsa is a very aggressive bug, which is why it's so dangerous.