Here's a key section (not sure if I was writing it that I would bother including figures for "chafing", though):
Indeed a vast, disturbing literature has now accumulated on the ill effects of running marathons. Studies find that up to 1 in 12 participants end up seeking medical help during the race. (At
hot-weather events, runners can end up “dropping like flies.”) As many as four-fifths report having gastrointestinal problems such as bloating, cramps, vomiting, diarrhea, and fecal incontinence while on the course. Some runners suffer from blood poisoning. Others must endure a blitz of dermatological conditions: sore nipples (affecting up to 1 in 6 on race day); chafing (another 1 in 6); blisters (1 in 3); and jogger’s toe (1 in 40). Given all the risks, it’s no wonder that some marathon organizers have asked doctors to embed as race participants so they can quickly tend to runners who collapse.
When researchers consider all the injuries that accrue during the period of training—and not just on the day of the marathon itself—they find even greater cause for alarm. One study looked at 255 participants in an extended, 32-week marathon training program and found that 90 of them—that’s 35 percent—experienced “overuse” injuries. (Among the most common training ailments are anterior knee pain, Achilles tendinitis, shin splints, and stress fractures.) Another research group surveyed 725 men who raced in the 2005 Rotterdam Marathon, and found that more than half of them had sustained a running injury over the course of the year. Among those who sustained a new injury during the month leading up to the race, one-quarter were still suffering, to some extent, three months later.