Piranha, small but dangerous?

Adult piranha from Amazon river

Many are asking especially when the movie Piranha came out, is that true that piranhas are so violent and dangerous. In fact that is true, especially if man is found alone in river.
Piranhas are found in South America, especially in Amazon river, and they are about 13cm to 25cm long. They attack in flock and that's the main reason why they are so dangerous. One piranha is probably no problem for man in the river, but as we mentioned, they always travel in flocks. Imagine one ant, it would be harmless, but few thousands of them should scare us. If you go to South Africa to vacation definitely aviod swimming in Amazon river.