Infection is a harmful, sometimes deadly response by your body to disease-causing microorganisms. Most infections are caused by bacteria, fungi or viruses. The pathogens enter your body and multiply, causing damage by producing toxins or by overgrowing and overwhelming your immune system. The infection may produce symptoms that range from mild and easily treatable to severe and life threatening.
Some infectious diseases spread through direct contact, like touching a doorknob that has germs on it and then putting your hands in your eyes or mouth. Others spread through the air, such as when someone who has a cold or the flu sneezes or coughs close to you. Still other infectious diseases are spread by bugs, including mosquitoes that carry the malaria parasite and deer ticks that spread Lyme disease. And some infections are spread by contaminated water and food, such as undercooked hamburger or unpasteurized fruit juice.
Your immune system fights most infections by killing the infectious organisms or by releasing substances that drive them out of your body. But some infections persist. The pathogens can survive in your deep tissues by generating antigens that don’t trigger an inflammatory reaction, or they can linger in the blood and cause inflammation and tissue destruction. Many of the factors that raise your risk for infections are linked to lifestyle changes, such as changing agricultural practices that expose people to new crop pests, or urbanization that concentrates large numbers of people into crowded conditions with poor sanitation.