Eggs are passed in the stool (stage 1), and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days.  The released rhabditiform larvae grow in the feces and/or the soil (stage 2), and after 5 to 10 days (and two molts) they become become filariform (third-stage) larvae that are infective (stage 3).  These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.  On contact with the human host, the larvae penetrate the skin and are carried through the veins to the heart and then to the lungs.  They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed (stage 4).  The larvae reach the small intestine, where they reside and mature into adults.  Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host (stage 5).  Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years.

Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle).  In addition, infection by A. duodenale may probably also occur by the oral and transmammary route. N. americanus, however, requires a transpulmonary migration phase.

Geographic Distribution:
The second most common human helminthic infection (after ascariasis).  Worldwide distribution, mostly in areas with moist, warm climate.  Both N. americanus and A. duodenale are found in Africa, Asia and the Americas.  Necator americanus predominates in the Americas and Australia, while only A. duodenale is found in the Middle East, North Africa and southern Europe.

Gravid proglottids are passed intact in the feces or emerge from the perianal region of the host (stage 1).  Subsequently they release typical egg packets (stage 2).  On rare occasions, proglottids rupture and egg packets are seen in stool samples.  Following ingestion of an egg by the intermediate host (larval stages of the dog or cat flea Ctenocephalides spp.), an oncosphere is released into the flea's intestine.  The oncosphere penetrates the intestinal wall, invades the insect's hemocoel (body cavity), and develops into a cysticercoid larva (stage 3).  The larva develops into an adult, and the adult flea harbours the infective cysticercoid (stage 4).  The vertebrate host becomes infected by ingesting the adult flea containing the cysticercoid (stage 5).  The dog is the principal definitive host for Dipylidium caninum.  Other potential hosts include cats, foxes, and humans (mostly children) (stage 6),(stage 7) .  Humans acquire infection by ingesting the cysticercoid contaminated flea.  This can be promulgated by close contact between children and their infected pets.  In the small intestine of the vertebrate host the cysticercoid develops into the adult tapeworm which reaches maturity about 1 month after infection (stage 8).  The adult tapeworms (measuring up to 60 cm in length and 3 mm in width) reside in the small intestine of the host, where they each attach by their scolex.  They produce proglottids (or segments) which have two genital pores (hence the name "double-pored" tapeworm).  The proglottids mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (stage 1).

Geographic Distribution:
Worldwide.  Human infections have been reported in Europe, the Philippines, China, Japan, Argentina and the United States.

Eggs are deposited on perianal folds (stage 1).  Self-infection occurs by transferring infective eggs to the mouth with hands that have scratched the perianal area (stage2).  Person-to-person transmission can also occur through handling of contaminated clothes or bed linens.  Enterobiasis may also be acquired through surfaces in the environment that are contaminated with pinworm eggs (e.g., curtains, carpeting).  Some small number of eggs may become airborne and inhaled.  These would be swallowed and follow the same development as ingested eggs.  Following ingestion of infective eggs, the larvae hatch in the small intestine (stage 3) and the adults establish themselves in the colon (stage 4).  The time interval from ingestion of infective eggs to oviposition by the adult females is about one month.  The life span of the adults is about two months.  Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area (stage 5).  The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions (stage 6).  Retroinfection, or the migration of newly hatched larvae from the anal skin back into the rectum, may occur but the frequency with which this happens is unknown.

Geographic Distribution:
Worldwide, with infections more frequent in school- or preschool- children and in crowded conditions.  Enterobiasis appears to be more common in temperate than tropical countries.  The most common helminthic infection in the United States (an estimated 40 million persons infected).

The unembryonated eggs are passed with the stool (stage 1).  In the soil, the eggs develop into a 2-cell stage (stage 2), an advanced cleavage stage (stage 3), and then they embryonate (stage 4); eggs become infective in 15 to 30 days.  After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae (stage 5) that mature and establish themselves as adults in the colon (stage 5).  The adult worms (approximately 4 cm in length) live in the cecum and ascending colon.  The adult worms are fixed in that location, with the anterior portions threaded into the mucosa.  The females begin to oviposit 60 to 70 days after infection.  Female worms in the cecum shed between 3,000 and 20,000 eggs per day.  The life span of the adults is about 1 year.

Geographic Distribution:
The third most common roundworm of humans. Worldwide, with infections more frequent in areas with tropical weather and poor sanitation practices, and among children. It is estimated that 800 million people are infected worldwide. Trichuriasis occurs in the southern United States.

Eggs are eliminated with feces or urine (stage 1).  Under optimal conditions the eggs hatch and release miracidia (stage 2), which swim and penetrate specific snail intermediate hosts (stage 3).  The stages in the snail include 2 generations of sporocysts (stage 4) and the production of cercariae (stage 5).  Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host (stage 6), and shed their forked tail, becoming schistosomulae (stage 7).  The schistosomulae migrate through several tissues and stages to their residence in the veins (stage 8, stage 9).  Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species (stage 10).  For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine (stage A), and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine (stage B).  However, both species can occupy either location, and they are capable of moving between sites, so it is not possible to state unequivocally that one species only occurs in one location.  S. haematobium most often occurs in the venous plexus of bladder (stage C), but it can also be found in the rectal venules.  The females (size 7 to 20 mm; males slightly smaller) deposit eggs in the small venules of the  portal and perivesical systems.  The eggs are moved progressively toward the lumen of the intestine (S. mansoni and S. japonicum) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively (stage 1).  Pathology of S. mansoni and S. japonicum schistosomiasis includes: Katayama fever, hepatic perisinusoidal egg granulomas, Symmers' pipe stem periportal fibrosis, portal hypertension, and occasional embolic egg granulomas in brain or spinal cord.  Pathology of S. haematobium schistosomiasis includes: hematuria, scarring, calcification, squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.
Human contact with water is thus necessary for infection by schistosomes. Various animals, such as dogs, cats, rodents, pigs, horses and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi.

Geographic Distribution:
Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East; S. haematobium in Africa and the Middle East; and S. japonicum in the Far East.  Schistosoma mekongi and S. intercalatum are found locally in Southeast Asia and central West Africa, respectively.

Life cycle of Taenia saginata and Taenia solium
Humans are the only definitive hosts for Taenia saginata and Taenia solium.  Eggs or gravid proglottids are passed with feces (stage 1); the eggs can survive for days to months in the environment.  Cattle (T. saginataT. solium) become infected by ingesting vegetation contaminated with eggs or gravid proglottids (stage 2).  In the animal's intestine, the oncospheres hatch (stage 3), invade the intestinal wall, and migrate to the striated muscles, where they develop into cysticerci.  A cysticercus can survive for several years in the animal.  Humans become infected by ingesting raw or undercooked infected meat (stage 4).  In the human intestine, the cysticercus develops over 2 months into an adult tapeworm, which can survive for years.  The adult tapeworms attach to the small intestine by their scolex (stage 5) and reside in the small intestine (stage 6).  Length of adult worms is usually 5 m or less for T. saginata (however it may reach up to 25 m) and 2 to 7 m for T. solium.  The adults produce proglottids which mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (approximately 6 per day).  T. saginata adults usually have 1,000 to 2,000 proglottids, while T. solium adults have an average of 1,000 proglottids.  The eggs contained in the gravid proglottids are released after the proglottids are passed with the feces.  T. saginata may produce up to 100,000 and T. solium may produce 50,000 eggs per proglottid respectively.

Geographic Distribution:
Both species are worldwide in distribution.  Taenia solium is more prevalent in poorer communities where humans live in close contact with pigs and eat undercooked pork, and very rare in Muslim countries.