HIV is a disease caused by the Human Immunodeficiency Virus. It's a virus that breaks down certain cells in your immune system and develops a disease called AIDS. Infectious diseases are diseases caused by organisms like viruses, bacteria, fungi, and parasites. These can cause familiar infectious diseases such as the common cold, flu, and warts. They also cause severe illnesses such as smallpox, cholera, diphtheria, Infectious diarrhea, etc.