Actually I've been thinking about this a little over the last few days, as there have been several threads on the subject. I reckon we've introduced a new cat(s) to the current feline and canine members of the family on 5 occasions (not many over 35 years?) and a new dog on 6. I've never agonised or theorised or taken any huge precautions - just done it. The most thought I gave to it was in getting the current boys.
I think worry increases to fit the time available and previously I've been juggling family job and pets and haven't had space to theorise. Never had any bother either.
As one of my favourite singers, Paul Simon, puts it - Maybe I Think Too Much
http://www.amazon.co.uk/gp/recsradio...k=004&disc=001