Which category of harmful agents can cause disease or irritation by pests?

The correct choice indicates that irritants are a category of harmful agents capable of causing disease or irritation by pests. These agents primarily cause discomfort and inflammatory responses when they come into contact with the skin, eyes, or respiratory system. In contexts associated with field sanitation, irritants may trigger allergic reactions or respiratory ailments, highlighting their potential to compromise health, especially for individuals exposed to them in agricultural settings or during pest control.

As for the other options, while asphyxiants can lead to suffocation by displacing oxygen in the environment, they do not primarily cause direct irritation. Anesthetics act to induce a loss of sensation or consciousness, which is not typical behavior associated with pest-related irritants. Systemic poisons are more focused on internal harm, as they enter the bloodstream and can affect various body systems; however, they don't specifically provoke irritation. Given this reasoning, irritants clearly fit the role of agents causing immediate discomfort and irritation from pests, aligning accurately with the characteristics described in the question.