As IggyB says, many people naturally carry it in their throats, and it can cause a mild infection in a healthy patient.
Experts have so far uncovered 17 strains of MRSA, with differing degrees of immunity to the effects of various antibiotics. Two particular strains, clones 15 and 16, are thought to be more transmissible than the others, and account for 96% of MRSA bloodstream infections in the UK.
It is a fact of life in the NHS that patients are at higher than normal risk of picking up such an infection on the wards. This is is for two reasons - firstly, that the population in hospitals tends to be older, sicker and weaker than the general population, making them more vulnerable to the infection.
Secondly, conditions in hospitals, which involve a great many people living cheek by jowl, examined by doctors and nurses who have just touched other patients, are the perfect environment for the transmission of all manner of infections.
In addition to causing endogenous (from colonisation to a wound) infections, MRSA can spread between patients, usually by direct or indirect physical contact. For example, hospital staff attending to a colonised or infected patient may become contaminated or colonised with MRSA themselves (perhaps only briefly). They may then spread the bacteria to other patients with whom they subsequently have contact. These patients may in turn become colonised and/or infected through cross-infection.
Some strains of MRSA that are particularly successful at spreading between patients may also spread between hospitals, presumably when colonised patients or staff move from one hospital to another. These strains are known as epidemic MRSA (or EMRSA for short).