70 people were injured in a piranha attack in Argentina on Christmas Day. While nobody was killed, one can imagine the panicked atmosphere present as bathers who were simply trying to cool off were suddenly involved in a fish biting frenzy. The question is, what sequence of events led to this happening? The media has implicated hot weather and fisherman waste as the inciting factors. Other authors have discussed similar attacks in the literature, and implicate damming of streams and rivers causing ideal spawning grounds for piranhas. These spawning grounds can then coexist with recreational areas, and this leads to the fish being in close proximity to people.
There is a large amount of folklore surrounding piranhas and their behavior. What is important to note is that the bites are typically defensive, and the fish are not feeding on humans (or other large mammals typically). The documented cases of piranhas eating humans involve people dying of other causes, then being eaten by the fish. The defensive nature of piranha bites is evident in that the most are single bites that are of a warning behavior. That bite can pack a punch, as one study demonstrated that piranhas bite up to three times harder than alligators proportionally.
This bite is typically circular and crater-like in nature, and damages skin and underlying tissues. The majority of these bites are on the lower extremities, which is not surprising. Most need minimal first aid, but occasionally bites can amputate digits or cause severe bleeding. They do not seem to need prophylactic antibiotics, but can rarely become infected with pathogenic bacteria, so the wound needs monitoring.
As far as mass attacks go, they typically occur during the major spawning seasons for piranha. They also occur most often when large numbers of people are in the water, and at shallow dammed sites. Because dammed sites can allow spawning year round, and allow larger groups of people to recreate together the attacks can be more common and larger in size. Thus, it seems that the attacks are fairly predictable, and not something that should cause paranoia.
Piranha attacks in dammed streams used for human recreation in the State of São Paulo, Brazil
Mega-Bites: Extreme jaw forces of living and extinct piranhas (Serrasalmidae)
Piranha attacks on humans in southeast Brazil: epidemiology, natural history, and clinical treatment, with description of a bite outbreak.