I worked in the NHS and it used to be said that each large hospital had its own strain, especially the large mental health hospitals.Also there appeared to be the issue of substrains in smaller geographical and social groups. The individual group may be immune but when groups mix, as in big hospitals, they exchange strains and, as a result, get ill. I was taught that as well as being spread by contact, it was also spread by droplets in the air and also by the faecal oral route, google flush plume. Working in the community, we didnt catch it nearly as much as our in patient colleagues even though we visited some pretty disgusting places and used to go from house to house. We were fanatical in handwashing and in the use of gel when we couldn't and that seemed to keep us safe