The correct answer is Virus. Explanation & Key Takeaways: • Chikungunya is a mosquito-borne disease caused by the chikungunya virus. • Viruses are small intracellular parasites, which contain either an RNA or DNA genome surrounded by a protective, virus-coded protein coat. • A complete virus particle is called a virion. • Viruses are inert outside the host cell and can be crystallized. • The main purpose of a virus is to deliver its genome into the host cell to allow its transcription and translation by the host cell. • A bacteriophage is a virus that destroys bacteria. Related Concepts: • Non-chlorophyllous bacterium - Bacteria are microscopic, single-celled organisms. They can be the cause of various diseases, but Chikungunya is not caused by any bacterial agent, whether it possesses chlorophyll or not. Chlorophyll is related to the ability of some bacteria (and plants) to perform photosynthesis, which is irrelevant to the conversation about the disease's causative agent. • Nematode - Nematodes, or roundworms, are a type of parasitic worm that can cause various diseases in humans, plants, and animals. • Fungus - Fungi are eukaryotic organisms that can be single-celled or multicellular. They can cause diseases in humans, known as mycoses, which can range from superficial, affecting the skin, to systemic, affecting multiple body systems.