What is Infection?

Infection

• Infection occurs when harmful agents, called pathogens, invade another organism and cause systemic or localized illness.
• Pathogens can be living organisms (bacteria, fungi, parasites) or non-living agents (viruses, prions).
• Pathogens can spread through various means, including blood, air droplets, mucous membranes, sexual intercourse, insect bites, a break in the skin, fomites, or any orifice.
• Infections can be transmissible within the same species (intraspecies) or between different species (interspecies).
• Not all infections are from interspecies or intraspecies contact; some infections come from environmental exposure, such as water, soil, or food.

Signs and Symptoms of Infection

• Signs of infection depend on the pathogen and may include redness, swelling, fever, abscess formation, myalgias, malaise, diarrhea, vomiting, difficulty breathing, impairment of body systems, and changes in pulse and blood pressure.
• Symptoms can be caused by the direct effects of pathogens or by the immune system's response to the pathogen.

Types of Infectious Agents

• Parasites: eukaryotes that can be unicellular and microscopic or macroscopic multicellular worms, with complex lifecycles.
• Bacteria: simple, single-celled, prokaryotic, microscopic organisms that multiply quickly by binary fission.
• Viruses: non-living, nanoscopic, infectious particles of DNA or RNA protected by a protein shell, which commandeers host cells to produce more viral copies.
• Fungi: eukaryotic, microscopic, unicellular organisms, such as yeasts, or macroscopic multicellular molds.
• Prions: infectious particles of misfolded proteins that biochemically coax normally folded proteins to malfunction, not alive and cannot reproduce.

Disease

• Disease occurs when some part of an organism cannot function properly, causing internal biochemical or structural imbalances with signs of illness.
• Any perturbation in normal functioning or homeostatic imbalance with signs of illness can be categorized as a disease.
• Diseases can be caused by internal processes (e.g., autoimmune reaction) or external factors (e.g., infectious microorganisms).
• Examples of diseases include rheumatoid arthritis, Huntington's disease, cancer, type 2 diabetes, and strep throat.

Infectious Disease

• Infectious disease occurs when infectious pathogens cause persisting localized or systemic symptoms.
• Infectious diseases occur when a harmful agent enters the body and cells from the innate and adaptive immune system are unable to clear the agent quickly.
• Signs and symptoms of infectious diseases vary based on the type of infectious agent, but most commonly involve fever, malaise, muscle pain, diarrhea, vomiting, and headache.

Examples of Infectious Diseases

• Salmonella, caused by the food-borne bacteria Salmonella enterica.
• Toxoplasmosis, caused by the parasite Toxoplasma gondii.
• Schistosomiasis, caused by parasites like Schistosoma haematobium.
• Scarlet fever, impetigo, or strep throat, caused by the bacteria Streptococcus pyogenes.
• Legionnaires disease, caused by the bacteria Legionella pneumophila.
• Meningococcal disease (meningitis or pneumonia), caused by the bacteria Neisseria meningitidis.
• Human immunodeficiency viruses (HIV), caused by a type of Lentivirus.
• Chicken pox, caused by the varicella-zoster virus.
• Creutzfeldt-Jakob disease (CJD), caused by a prion.
• Chlamydia, caused by the sexually transmitted bacteria Chlamydia trachomatis.

Disease vs. Infection

• The difference between infection and disease is rooted in its etiology, or causal factor.
• Infections and infectious diseases are always caused by invading pathogens contracted from other organisms or the environment.
• Not all diseases are caused by pathogens; diseases can also be caused by internal processes or genetic mutations.
• The immune system's response to infectious disease and noninfectious diseases is different.

Types of Disease

• Infectious disease
• Genetic disease
• Metabolic disease
• Autoimmune disease
• Autoinflammatory disease
• Allergic diseases
• Neoplastic disease
• Primary immune deficiency diseases

Opportunistic Infections

• Opportunistic infections are caused by microorganisms that do not normally cause disease in a person with a properly functioning immune system.
• Examples of opportunistic infections include Pneumocystis jirovecii pneumonia in someone with AIDS or an atypical Mycobacterium infection in someone with a primary immune deficiency.

Learn about the process of infection, its causes, and the concepts of pathogens, infectivity, and transmission.