Nettie remember it's air, water and soil borne ... hubby could have got it via anything and the main thing is, he has been treated and he has you to keep an eye on him.
Here , I have copied and pasted this for you , it's a basic understanding of what it is...
http://www.wisegeek.com/what-is-pseudomonas-aeruginosa.htm
Pseudomonas aeruginosa is an opportunistic bacteria that lives in soil, water, and even in environments like hot tubs. For most healthy people, this bacteria seldom poses a problem. Occasionally people will develop conditions like hot tub rash, and swimmer’s ear, which may be due to contact with these germs. These conditions can sometimes resolve without treatment, or with minimal treatment, like antibiotic drops for swimmer’s ear.
Pseudomonas aeruginosa tends to live all around us, it is most dangerous to those who are in weakened physical states or have immunodeficiencies. Despite hospital cleaning and safety the bacteria may aggressively survive in basic hospital equipment, like masks used to give oxygen, breathing apparatus, or catheters for urine.....
Interesting to note that even with careful cleaning it can still survive, it's all around us ..it sounds like a blasted nuisance !
Big hugs
Winnie