Infectious diseases are caused by infectious organisms. Typically, these are bacteria, viruses, fungi, or worms/ helminths. Under normal circumstances, when the immune system of the host is fully functional, disease symptoms may not develop. If the host immune system is compromised, or the infectious agent overwhelms the immune system, an infectious disease ensues. Most infections are caused by bacteria, viruses, protozoa, helminthes, rickettsia, and fungi.
Infectious diseases can be caused by many pathogens, including bacteria, viruses, fungi, and parasites that may cause illness and disease. For humans, transmission of pathogens may occur in a variety of ways: spread from person-to-person by direct contact, water or foodborne illness or aerosolization of infected particles in the environment and through insects mosquitos and ticks.
Signs and symptoms and treatment of infectious diseases depend on the host and the pathogen.
Infectious diseases in humans are caused by microorganisms including:
Infectious diseases spread in multiple ways. In many cases, direct contact with a sick individual, either by skin-to-skin contact (including sexual contact) or by touching something another person touches, transmits the disease into a new host. Contact with body fluids, such as blood and saliva, also spreads infectious diseases.
Some diseases spread through droplets discharged from a sick person’s body when they cough or sneeze. These droplets linger in the air for a short period of time, landing on a healthy person’s skin or inhaled into their lungs.
In some cases, infectious diseases travel through the air for long periods of time in small particles. Healthy people inhale these particles and later become sick. Only certain diseases spread with airborne transmission, including tuberculosis and the rubella virus.
Symptoms of infectious disease are particular to the type of disease. For example, symptoms of influenza include:
Other infectious diseases, such as Shigella, cause more serious symptoms, including:
You may experience one or several symptoms of an infectious disease. It’s important to see a doctor if you have any chronic (ongoing) symptoms or symptoms that get worse over time.
Doctors diagnose infectious diseases using a variety of laboratory tests. Samples of blood, urine, stool, mucus or other body fluids are examined and provide information used in the diagnostic process.
In some cases, doctors identify infectious organisms by examining them under a microscope. Occasionally, laboratories must grow, or culture, the infectious organism from a sample to confirm its presence.
Treatment depends on which microorganism causes the infection.
In all cases, doctors treat specific symptoms of infectious diseases according to the latest medical guidelines. Talk with your doctor about your symptoms to explore possible treatment options.
You can also reduce your risk of contracting an infectious disease by: