What type of agents are defined as substances used for hostile purposes to cause disease or death?

The correct type of agents defined as substances used for hostile purposes to cause disease or death are biological agents. Biological agents include pathogens such as bacteria, viruses, fungi, or toxins that can be utilized in attacks to harm individuals or populations by causing illness or death. These agents can be naturally occurring or engineered, and their purpose in a hostile context makes them particularly concerning in discussions of bioterrorism and public health threats.

On the other hand, explosive agents are substances designed to create an explosion, while chemical agents typically refers to toxic chemicals that can cause harm through exposure rather than organisms that induce disease. Nuclear agents relate specifically to radioactive materials and their harmful effects from radiation rather than biological impacts directly from infectious agents. Therefore, biological agents are the most accurate answer when discussing substances used intentionally to inflict harm through disease or lethal effects.