Some parasitic species cause serious diseases in human beings and other hosts. They have slender, round bodies with tapered ends. Roundworms have remarkable powers of reproduction and are extremely numerous. Researchers have found more than 90,000 roundworms in a single rotting apple. Nearly all species of roundworms reproduce by laying eggs.
Some species produce great quantities of eggs.
For example, females of the species Ascaris lumbricoides each lay about 200,000 eggs per day for at least 10 months. Among some species of roundworms, the eggs hatch into tiny young that look like the adults. Eggs of other species hatch into young called larvae, which gradually transform into adults. Species of roundworms that do not lay eggs give birth to larvae. Parasitic roundworms may infect a host in a number of ways.