Viruses are very small organisms living on the Earth and can infect both plants and animals. The three diseases caused by viruses and their mode of communication are as follows:

(i) Dengue: It is a viral disease caused by dengue virus and transmitted by Aedes mosquito. The symptoms for this disease includes high fever, headaches, back and joint pain, rashes and eye pain.

(ii) AIDS: Aquired immunodeficiency syndrome or AIDS is a viral disease caused by Human Immunodeficiency Virus or HIV. It is a sexually transmitted disease which means it can be transmitted through exchange of body fluids like saliva, semen or even blood.

(iii) Rabies: A viral disease spread to people from the saliva of infected animals. Symptoms of the disease may include fever, fatigue, dizziness, nausea, vomiting, headache, excess salivation, muscle spasms, paralysis and mental confusion.