Megaklothovirus horridgei is the largest virus ever discovered. It is 3.9 μm long, which is about twice the length of an Escherichia coli bacterium. It was first discovered in 2018 on an arrow worm of the genus Spadella that it was infecting. Before Megaklothovirus horridgei was understood to be a giant virus, it had been mistaken for bristles in 1967, and later for bacteria in 2003.