Which of the following organisms can negatively impact the well-being of humans and animals?

Fleas can significantly impact the well-being of humans and animals, making them the correct answer. These parasites feed on the blood of their hosts, which can lead to discomfort, itching, and allergic reactions. In addition to causing pain and irritation, fleas can transmit various diseases, including the bubonic plague and cat scratch fever, which are harmful to both pets and humans.

While ants, bees, and spiders can interact with humans and animals, their effects vary. Ants may occasionally bite or sting, but they are not typically harmful unless in large numbers or if they are aggressive species. Bees can pose a risk primarily through stings, especially to those who are allergic, but they also play crucial roles in pollination and ecosystems. Spiders can be beneficial by controlling other pest populations, although some species possess venom that can harm humans. However, the overall impact of fleas, particularly regarding direct negative consequences on health and comfort, is more pronounced, emphasizing their role as a significant concern in pest management.