New York City has 600K+ dogs and 400K+ cats; and a zoo in Central Park. New York City playgrounds were found contaminated with toxocara (roundworm), which lives in the intestines of dogs and cats, particularly stray animals. Toxacara can infect the liver, lungs, brain, eyes, and central nervous system; and has been linked to a reduced IQ.Metropolitan areas in Brazil have a highly density of pets, including dogs, cats, birds, and monkeys; 50% of elementary school children and 60-80% of pregnant women have toxoplasmosis (cats); and the pathogens in pets closely match the pathogens in pet owners.

Animals are a vector for infectious pathogens, from animal-to-animal, animal-to-person, and person-to-animal. Underlying chronic infection with pathogens originating in animals can cause chronic diseases—and can exacerbate acute viral illnesses, such as influenza and covid-19.