Helminthiases, the vast variety of disease of man and animals caused by an infestation of worms, were formerly considered to be limited to the tropical belt, but increasing travel and trade has caused them to spread around the world. The pathogenic effects may not be apparent for a long time, making accurate diagnosis difficult. Infestation occurs in several ways, depending on the type of worm: consumption of vegetables and fruits contaminated by soil or dirty hands; consumption of inadequately cooked foods, including meat and fish; punctures in the skin caused by the bites of blood-sucking insects; or contact with contaminated soil by going barefoot or lying on the ground (some larvae are able to actively penetrate the skin).
There are 3 groups of helminths: trematodes (flukes), cestodes (tapeworms and hydatid worms), and nematodes (roundworms including ascarids).
Intestinal helminthic infections affect many millions of people in Africa, Asia, and Latin America. Development projects have increased the incidence of many of these diseases because drainage and irrigation canals, large water impoundments and artificial man-made lakes provide habitat for certain of the parasite hosts.
An estimated 25 million Americans, many of them young children and many from middle class and of affluent families, are often unknowing hosts to tiny intestinal worms.