It is the most common parasite in human intestinal tract, mostly in children, parasitic in small intestine, mostly in jejunum, and mainly eats semi-digested food.

The adult of Ascaris lumbricoides is cylindrical, much like an earthworm, with pale yellow and pink stripes on its body surface. If it's a male, the tail will curl. When male and female adults mate to lay eggs, the eggs are usually excreted with feces.

Ascaris lumbricoides is a worldwide species and the most common parasite in human body. The infection rate can reach more than 70%, higher in rural areas than in cities, and higher in children than in adults.

Life history of ascaris lumbricoides;

Adults are parasitic in the small intestine, mostly in the jejunum, and feed on semi-digested food. After mating between females and males, females lay eggs and the eggs are discharged with feces, which pollutes the environment. Under the conditions of cool, humid, sufficient oxygen and suitable temperature (2 1~30℃), the fertilized eggs develop into first stage larvae after two weeks, and become infected eggs after molting for the first time.

During the infection period, eggs are swallowed and larvae hatch in the small intestine. Larvae can secrete hyaluronidase and protease, invade intestinal mucosa and submucosa, drill into small veins or lymphatic vessels of intestinal wall, enter the liver through veins, then pass through the right heart to the lungs, break through capillaries and enter alveoli, where they molt for the second and third time.

Then, it moves along the bronchus and trachea to the pharynx, is swallowed by the host, reaches the small intestine through the esophagus and stomach, molts for the fourth time in the small intestine, and develops into an adult several weeks later. It takes about 2 months from the time when infected eggs enter the human body to the time when females begin to lay eggs. The life span of adults is about 1 year, and each woman ovulates about 240,000 times a day. The number of adults in the host is generally one to dozens, and individual adults can reach thousands.