Toxicology is the study of poisons. It focuses on the harmful effects that chemicals, substances, or toxins can have on living organisms. Toxicologists investigate how these poisons interact with the body, their effects on different organs and systems, and the mechanisms of toxicity. By understanding the nature of poisons and their impact, toxicologists can identify potential hazards and develop strategies to prevent or mitigate their harmful effects on human health and the environment. While viruses, bacteria, and diseases can all have harmful effects on the body, toxicology specifically examines the toxicity of substances, chemicals, or toxins that can cause harm. Therefore, the correct answer is option 4 - poisons.