I disagree about Aikido being the least injurious martial art. Not by a long shot. There's Tai Chi, which is unlikely to get you anything worse than a mild knee overuse injury. There are many Karate/TKD schools which only allow light sparring, so the worst you're looking at there is probably something like a pulled muscle or an accidental black eye. Lots of Kendo and Kenjutsu-type arts are surprisingly safe, despite the ever-present risk of getting cracked with an oaken stick.
I think Aikido is actually one of the most injury-prone arts this side of serious NHB/full-contact type stuff in practice. Falling down repeatedly involves a lot of wear and tear, as well as risks involved with landing improperly. Knee-walking/suwari-waza is one of the worst activities for long term knee health I've ever seen. And, as Justin observed, the upper limb joints are often being manipulated and torqued in extreme ways and even at ballistic speeds.
Nonetheless, if one is in good general shape and/or devotes consitent effort to injury-preventative conditioning, I don't think the risks of Aikido are prohibitive. The most important thing in the long run is to listen to the pain signals given by the body, giving rest and recuperative attention where it is due.
In terms of avoiding death or paraplegia, the most important thing is to avoid backward rolls and avoid practicing under conditions of extreme fatigue.