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Roundworms are large intestinal nematodes and among the most prevalent parasites on Earth. Adult worms live in the small intestine and can reach 15–35 cm in length. Eggs are shed in feces and can survive in soil for years. In pets, Toxocara species are the most common intestinal parasites and can occasionally infect humans (visceral larva migrans).

Tapeworms are flat, ribbon-like segmented worms that attach to the intestinal wall using suckers and hooks. They have no digestive system and absorb nutrients directly through their skin. Adult worms shed egg-filled segments (proglottids) that look like grains of rice. Some species (Echinococcus) can form dangerous cysts in organs.

Giardia is a microscopic flagellated protozoan that colonizes the small intestine. It exists in two forms: the active trophozoite (pear-shaped, with two nuclei resembling a face) and the hardy cyst form that can survive in cold water for months. It is a leading cause of waterborne diarrheal illness worldwide.

Toxoplasma gondii is an obligate intracellular parasite that can infect virtually any warm-blooded animal. Cats are the only definitive host where the parasite reproduces sexually. In most healthy humans, infection causes no symptoms. However, it poses serious risks to pregnant women (congenital toxoplasmosis) and immunocompromised individuals.

Heartworm is a serious, potentially fatal parasitic disease caused by foot-long worms living in the heart, lungs, and pulmonary arteries. Dogs are the natural host and can harbor hundreds of worms. Cats are atypical hosts — they usually have fewer worms but the disease is often more sudden and severe. Transmitted exclusively by mosquitoes.

Lice are tiny wingless insects that live on the skin and hair of their host. They are highly host-specific — human lice cannot survive on pets and vice versa. Human head lice (Pediculus humanus capitis) are the most common, especially in school-age children. They feed on blood and lay eggs (nits) cemented to hair shafts close to the scalp.

Fleas are wingless, laterally compressed insects that are extraordinary jumpers — they can leap 150 times their own body length. The cat flea (Ctenocephalides felis) infests both cats and dogs. Only 5% of the flea population lives on the pet — 95% (eggs, larvae, pupae) are in the environment. A single female can lay 2,000 eggs in her lifetime.

Ticks are blood-feeding arachnids (related to spiders and mites) that are important vectors of numerous serious diseases. They cannot jump or fly — they 'quest' by holding onto vegetation with their back legs and reaching out with their front legs to grab passing hosts. Hard ticks (Ixodidae) feed for days; soft ticks (Argasidae) feed quickly.

Hookworms are small (1–1.5 cm) but voracious blood-feeders. They attach to the intestinal wall with cutting plates or teeth and feed continuously, secreting anticoagulants to keep blood flowing. A single Ancylostoma duodenale can consume 0.2 mL of blood per day. Heavy infections cause severe iron-deficiency anemia, particularly dangerous in puppies, kittens, and children.

Scabies is caused by microscopic burrowing mites that tunnel into the outer layer of skin (stratum corneum) to lay eggs. The intense itching is an allergic reaction to the mites, their eggs, and feces. In dogs, the same genus causes sarcoptic mange — a severe, highly contagious skin disease. Human and dog strains can cross-infect but do not establish permanent infestations in the non-primary host.