Infectious diseases are illnesses caused by microorganisms such as viruses, bacteria, fungi, or parasites, which are collectively called pathogens. These diseases can spread from one person to another or through other means such as contaminated food, water, insect bites, or contact with infected surfaces. Pathogens cause infectious diseases by either disrupting the body's normal functions or triggering the immune system to respond, leading to symptoms like fever and inflammation. Infectious diseases can be spread through various ways including direct contact with bodily fluids, aerosols (like coughs or sneezes), vectors like mosquitoes, or by touching contaminated objects. Examples of infectious diseases include respiratory infections (like colds and flu), stomach flu, sexually transmitted infections, and fungal infections like athlete's foot. These diseases vary in severity from mild to life-threatening and are among the leading causes of death worldwide. Infectious diseases are distinct from noninfectious diseases, which are caused by genetic, environmental, or lifestyle factors and cannot be transmitted between people. Treatments depend on the type of pathogen involved, such as antibiotics for bacterial infections, antivirals for viral infections, antifungals for fungal infections, and antiparasitic drugs for parasitic infections. Preventive measures include vaccination, good hygiene practices like handwashing, and avoiding exposure to infectious agents.