[gemy]

I do second that advice. It could be anything physical that is causing pain and creating a "nasty disposition". An infected gum line, tooth decay, uti, etc.

Sudden behavioural changes usually signify a physical problem. But sometimes they are really not so sudden, but have slowly built up over time.
It is just we dismissed this response or that response.

The key is to rule out physical problems. Then to address the behavioural issues.

There are some good training video links, posted in the Training forums. You could do a search for "dog aggression to ppl"