Communicable diseases, also known as infectious diseases, are illnesses caused by pathogenic microorganisms such as bacteria, viruses, fungi, and parasites. These diseases can spread from one person to another through direct or indirect contact, including air, food, water, or bodily fluids. Examples of communicable diseases include the flu, measles, tuberculosis, HIV/AIDS, and COVID-19. The spread of communicable diseases can be prevented through measures such as vaccination, good hygiene practices, and isolation or quarantine of infected individuals.