Question about quarterback durability?

Perhaps Brett Favre's most remarkable statistic from all his years playing is his 256 consecutive starts, which have included over 450 sacks, plus countless other abuses by defenses through the years. Why is he so durable? Is it just luck or does it have something to do with his body? Why do some bigger quarterbacks like Donovan McNabb (240 lbs) seem to get injured every year?