I've felt pretty safe in the ED. It's not that things don't happen (once saw a guy cut himself out of 4 point leathers with a 5" knife in the ED), and I've certainly been in my share of scrapes, but in general, there is comfort in the fact that there are always people around. You might have to hold someone down as you get the restraints on (as docB says, while patients are flailing around) and may get smacked once or twice. There certainly are incidents where a patient gets a good swing or kick in and can do some damage, but there's almost always help when you need it. On the 'bolance, things are a little different. I have wrestled alone with several patients on the scene or in the truck (including one guy with an 8" knife), intubated belly down in the street during a riot, and found myself in some very unruly crowds. While the ER is not exactly a "controlled environment", the chances of you getting killed there (by someone other than the nurses or physicians providing care) are small.
Experience is a good teacher in these things. If a hispanic man gets very quiet and takes his hat off, he's getting ready to fight. If the patient can't take their eyes off the door, they're getting ready to run. A person without physical disability who walks with one arm swinging but the other one close to their waist (without being hooked on the belt or in the pocket) is armed. Physical and psychological clues will tell you a lot, and your instincts will save your butt many times. Patients are very rarely unpredictable when they become assaultive. Usually there is a lead-up period of escalating physical activity, voice, and emotion before they start swinging. Also beware any sudden change in mood. The annoyed patient who suddenly becomes very quiet or very cooperative is up to something. EMIMG has a good point about not putting yourself in unsafe situations. Don't let the patient get between you and the door. Leave the door open if you suspect a patient may become a problem for you. If a patient is suicidal, they sure don't care about your life either. If they came in wearing handcuffs, the cuffs stay on.
As far as the contagious diseases go, it's perhaps only a little bit more likely in the ER than elsewhere that you'll catch something. Just remember to put a mask on the cachectic homeless HIV drug abuser with the unstoppable cough. Waiting for your PPD to convert is no fun at all.
'zilla