While many types of jellyfish are relatively harmless to humans, some can cause severe pain and are more likely to cause a systemic reaction. These jellyfish cause more-serious problems in people: Box jellyfish. Box jellyfish can cause intense pain. Jellyfish are over 600 million years old. Before the dinosaurs, trees, or even fungi, there were jellyfish. There could be 300,000 species of jellyfish. They have no brain and are 98% water. In their ecosystem, jellyfish are effective predators.