Infectious diseases are illnesses caused by microorganisms like bacteria, viruses, fungi, or parasites that invade the body, multiply, and can be transmitted directly or indirectly between people, animals, or through the environment. Common types include respiratory infections (COVID-19, flu), gastrointestinal illnesses, and blood-borne pathogens (HIV/AIDS). Symptoms often include fever, fatigue, and pain, with treatment ranging from rest to antibiotics or antivirals.