While attacks on humans by piranhas are rare, they do occur. In addition, other predators in the Amazon River, such as caimans and anacondas, can also pose a threat to swimmers.

The Amazon River presents a diverse array of biological and environmental dangers for travelers and locals alike. Biologically, the river is home to black caimans, which are formidable apex predators, and bull sharks, which can swim thousands of miles upstream into freshwater. While often sensationalized, piranhas can be a threat in stagnant water during the dry season, and electric eels can deliver shocks strong enough to stun a human. Smaller "invisible" threats are often more dangerous, such as the candiru (a parasitic catfish) and the risk of insect-borne diseases like malaria, yellow fever, and dengue carried by mosquitoes near the water's edge. Environmentally, the river's massive discharge creates strong, unpredictable currents and whirlpools that can easily capsize small boats. In 2026, travelers are also warned about "microscopic threats," as untreated river water contains heavy loads of bacteria and parasites that can cause severe gastrointestinal illness if accidentally ingested during swimming.