So actually, piranha aren't as dangerous as most people think. There have been a handful of fatal attacks in recorded history, but they're generally when people can't get to shore. If this woman fell in, it would be unpleasant when she got a bunch of tiny bites, but as long as she didn't go for a long swim, she would be fine.

The folklore of the killer piranha swarms comes from Teddy Roosevelt. He was in the Amazon, and the locals, wanting to impress him, caught a massive amount of piranha, kept them cordoned off without feeding them, and then when Roosevelt was watching, they threw a cow in to be eaten. The sheer volume of piranha combined with their starvation caused a huge feeding frenzy and the cow was devoured. Roosevelt mentioned that feeding frenzy in a book he wrote, and from that point on piranha were known as killers. Realistically though, they don't pose much of a threat to humans beyond the pain of being bitten.