Giardia lamblia, a protozoan (cyst stage) found in water contaminated by animal faeces, coloured scanning electron micrograph (SEM). Causes the protozoal infection known as giardiasis. Parasitic protozoan that causes giardiasis (Giardia lamblia). . Giardia lamblia, a human parasite of the gastrointestinal tract. The organism is spread by direct contact or through contaminated food and water. Giardia spp. are pear-shaped, with hair-like flagella for motility. They cause the disease giardiasis (or lambliasis), an infection of the small intestine most common in tropical areas. Giardia spp. attaches by means of sucking discs to microvilli in the human intestine. Abdominal cramps, swelling, diarrhoea and nausea may occur. Magnification: x1, 405 when shortest axis printed at 25 millimetres.