Infectious diseases are caused by pathogens like bacteria, viruses, fungi, or parasites. While many microbes coexist harmlessly with humans as part of the normal flora, some can invade the body through direct or indirect contact, contaminated food or water, or vectors like mosquitoes. Pathogens use virulence factors to invade tissues or evade immune defenses, sometimes causing inflammation that damages host tissues. Symptoms vary by infection site, with fever being a common response. Diagnosis often requires lab tests, and treatment depends on the pathogen type. Preventative measures like hygiene, vaccines, and prophylactic antibiotics are crucial in reducing transmission.