Aflatoxins are toxins that are produced by fungi. Fungi are microorganisms that can grow on crops such as peanuts, corn, and tree nuts. These toxins are particularly prevalent in warm and humid climates and can contaminate food and feed products. Aflatoxins are known to have harmful effects on human and animal health, as they are carcinogenic and can cause liver damage. It is important to prevent or reduce the presence of aflatoxins in food and feed through appropriate storage and processing techniques. While bacteria, algae, and viruses can also produce toxins, in the case of aflatoxins, they are specifically attributed to fungi. Therefore, option 4, fungi, is the correct answer.