I have had a flock for more than the last 5 years, and periodically before that. I think it depends on what you are going to do.... there is a huge difference between 5 and 50 + birds.
A small flock, I generally have about 10 head.... is less expense in feed, gives me more than enough eggs for my family. However, while we do have an occasional rooster to dinner, most of my chicken to eat, I buy. But also, it is a small enough flock, that (God forbid) if I did get a disease, it would not be a big financial matter to start over. I don't vaccinnate. I don't sell birds to others, and while I have brought in other birds, I don't do it real often, and have been quite lucky as in nothing ever brought in a disease.
Now if you are going to do this seriously as in large flock, you are risking quite a bit more financially and need a more stringent plan.
As for raising healthy birds - plenty of space, clean water, good quality feed, good ventilation will produce darn healthy birds, as long as you start with healthy robust birds. Some chicks die, and I let them. I want a more hardy bird in my flock. Many on here go to extremes to keep birds alive, but I don't, I do think that can infect your whole flock. But in years, while I have lost plenty to predators, but none to illness. I finally have the predator thing pretty well handled.
Good luck, hope this helps, mrsK