What is a primary health concern regarding nuisance flies?

The primary health concern regarding nuisance flies is their ability to carry disease-causing organisms. Flies are known vectors for various pathogens, including bacteria, viruses, and parasites. As they move from decaying organic matter, waste, or contaminated surfaces to food and food preparation areas, they can transfer these harmful organisms, which poses significant health risks to humans. Diseases associated with nuisance flies can include gastroenteritis, food poisoning, and other infections, making their presence particularly problematic in areas where food is present or where sanitation may be compromised.

The other choices do not accurately represent the primary concern. While allergic reactions among older adults can be a consideration in certain contexts, it is not the primary health risk associated with nuisance flies. The suggestion that flies deplete oxygen levels in water is incorrect, as flies are not linked to changes in oxygen levels in aquatic systems in this way. Additionally, while flies can be a nuisance during colder months when they seek shelter, this is more related to their behavior than a direct health concern.