The correct answer to the question is option 2: virus. Rabies is a viral disease that affects the nervous system of mammals, including humans. The virus responsible for causing rabies is called the Rabies lyssavirus. It is transmitted through the saliva of infected animals, usually through bites or scratches.

Option 1: Bacteria is not the correct answer. Bacteria are single-celled organisms that can cause various diseases, but rabies is specifically caused by a virus, not bacteria.

Option 3: Fungus is also not the correct answer. Fungi are a separate category of organisms and are not responsible for causing rabies.

Option 4: Algae is not the correct answer either. Algae are photosynthetic organisms found in aquatic environments and are not associated with the transmission or development of rabies.

Therefore, the correct answer is option 2: virus.