Infectious disease refers to illnesses caused by harmful microorganisms, such as bacteria, viruses, fungi, or parasites. These tiny organisms can invade the body, multiply, and disrupt normal functions, leading to various health issues. Common examples include the flu, common cold, and COVID-19. Transmission of infectious diseases can occur in many ways, including through airborne particles, contaminated surfaces, or direct contact with an infected person. Preventive measures, such as vaccinations, good hygiene practices, and proper sanitation, are essential in controlling the spread of these diseases and protecting public health.