Intestinal parasites are organisms that live in the human gastrointestinal tract, making a home and exploiting the host for food and shelter. They come in various forms—from worms (helminths) to single‑celled organisms (protozoa)—and are responsible for a number of health problems worldwide. Although these parasites may be small, their presence can lead to significant discomfort and, in some cases, serious health complications. In this article, we will provide an in‑depth look at intestinal parasites, including how they are transmitted, the many ways they affect health, and the methods available both for treatment and for prevention.

I. An Introduction to Intestinal Parasites
A. What Are Intestinal Parasites?
Intestinal parasites are a broad group of organisms that infect the gastrointestinal tract of humans. They include multicellular worms—such as roundworms, tapeworms, and hookworms—and single‑celled protozoa, such as Giardia and Cryptosporidium. Although these organisms vary greatly in size, structure, and life cycle, they all share a fundamental characteristic: they live in the intestines of the host, extracting nutrients from the food the host consumes.

These parasites are not new to humans. Throughout history, intestinal parasites have been a constant in many populations, particularly in regions with suboptimal hygiene practices or inadequate access to clean water. Even in developed countries, intestinal parasitic infections can occur, particularly among travelers, in crowded living conditions, and as a consequence of inadequate sanitation.

B. How Do They Live and Multiply?
Parasites are masters of survival. They take advantage of the human body by using it as a source of nourishment and a safe environment to multiply. For example, many intestinal worms attach themselves to the wall of the intestines, absorbing nutrients directly from digested food. Protozoan parasites, on the other hand, multiply rapidly by cellular division and produce cysts that are excreted in the host’s feces.

The life cycle of these parasites typically involves several stages, with some forms requiring an external environment to develop before infecting another host. This ability to survive outside the body for periods—often as infective cysts—enables them to spread from one person to another, making their control and prevention a persistent public health challenge.