It's just a weird thing with the cartilage (if someone knows what I'm talking about and I'm screwing it up, please correct me *my doctor didn't really explain it*) in between the bones. Pressure or friction is put on it from the bones and it gets inflamed; wear and tear makes it damaged and painful.
If you have it when your bones are still growing and you let it heal, you're good to go. The problem with a 17/18 year old female is that your bones are pretty much done growing, so you don't know whether it will stay with you for life or not.
They don't know what causes it and they don't know how to cure it, other than cutting out all physical activity and stress for several months in the hope that it will go away (and stay that way). They've just told me to work on ankle strengthening and ice it if it hurts (since there's nothing else that I can do if I actually want to be active).
For the majority that don't know, I had multiple stress fractures in the tallus and navicular bones in BOTH of my feet. This started in Feb. 2005 and lasted just about until now. They had no idea what it was until late Nov. 2005, and then they only thought that it was the right foot (a bone scan showed significantly increased blood flow to the same spot on the left foot as in the right, but they thought that it was swelling from compensating for my right). I wore a boot on my right for three months (including to Summit) and then tried to get back to vaulting in time for my junior season. Pain returned in both feet, this time greater in the left. They still didn't know what was wrong, so they told me to not do anything and to try wearing a boot on my left foot for a month and a half to see if it would help. It didn't, and let me tell you that the Sac State stadium stairs were not meant for people with boots

.
Around that same time (mid-April 2006), I noticed some pain in my back (I had still been doing core and upper body strengthening in addition to rope drills so that I wouldn't be quite so behind when I was able to vault again) that, over a two week period despite not doing anything, progressed from a slight pinch when I did bridges to not being able to sit/stand/walk/bend/move my back without INTENSE pain. They couldn't tell me what was wrong (I think that I went to over a dozen doctors and specialists for these two injuries), but in August they assumed that it was broken and told me that I would have to wear a back brace for three months. A "let's just check" bone scan showed no break, so the diagnosis was changed to "pain from unknown cause." I had to cut out all physical activity (you don't have that much of a choice with the type of pain and limited movement that I had) from June to the end of October (talk about hard!), and was directed to a physical therapist specializing in back injuries.
Three weeks ago I was able to jog, and this morning I got the ok to start vaulting again. After 12 years of gymnastics (also ended by injuries) I vaulted 10'6" my freshman year (I'm a girl, if you haven't picked that up) with absolutely no form and beat my school's frosh record by a foot. I got a lot stronger, faster, and technically aware in the offseason and when I went to Summit came down with the flu. Two weeks later, on the first day of my sophomore season of track, I noticed the pain in my feet. Those injuries eliminated my sophomore and junior seasons, but I kept going to practices and meets (baking treats and helping out with some coaching *even though our coach is amazing* with everything that I learned here). Now, for my last season, my goals are to 1. stay healthy and 2. enjoy every second of it.