Do jellyfish harm humans

Yes, jellyfish can kill humans if they are big enough. If it just a wee lad than it will only sting.

Also, the most dangerous jellyfish is called the Australian box jellyfish, the venom is the most deadly in the animal kingdom, it has caused at least 5.568 recorded deaths since 1954. Each tentacle has about 500,000 cnidocytes, harpoon-shaped needles that inject venom into the victim.