Infection facts
• Infection is a host’s response to a pathogen.
• Viruses, bacteria, fungi, and parasites cause infection.
• Infection results when a pathogen enters the body; the pathogen attaches to a cell and destroys the cell’s protective membrane, spreads through blood and lymph, multiplies, and causes infection in target organ or tissue.

Understanding infection
• Results when a host organism responds to a pathogen or disease-causing substance.
• Develops when tissue-destroying microorganisms enter and multiply in the body.
• Takes the form of minor illnesses, such as colds and ear infections, or results in a life-threatening condition called sepsis, which causes widespread vasodilation and multiorgan system failure.
• Caused by four types of microorganisms:

How infections develop
Infection occurs when the body’s defense mechanisms break down or when microorganisms override the defense system. Other factors include:
• poor nutrition
• stress
• humidity
• poor sanitation
• crowded living conditions
• pollution
• dust.
Infection results when a pathogen enters the body through direct contact, inhalation, ingestion, or an insect or animal bite.

Understanding infectious disorders
Herpes simplex — virus fuses to host cell membrane, turns off host’s protein synthesis, and replicates; and viral deoxyribonucleic acid is released.
Herpes zoster — reactivation of varicella-zoster virus; trigger of reactivation unknown.
Infectious mononucleosis — virus invades B cells of oropharyngeal lymph; B cells die, and virus is released in blood; fever and other symptoms occur.
Lyme disease — tick injects spirochete into blood or deposits feces on skin; after incubation, spirochetes migrate, cause a rash, and disseminate to other skin areas and organs.
Rabies — bite transmits virus through skin; the virus replicates and spreads to the spinal cord and brain where it replicates again.
Respiratory syncytial virus infection — virus transmitted through respiratory secretions; bronchiolitis or pneumonia occurs (severe cases may lead to bronchiolar epithelial damage); and alveolar spaces may fill with fluid.
Rubella — virus transmitted through body fluid contact; the virus replicates in respiratory tract and spreads to the bloodstream.
Salmonellosis — a gram negative bacilli that is ingested through contaminated food or water, or through contact with an infected person or animal; the bacilli then invades the upper small bowel and causes bacteremia.
Toxoplasmosis — intracellular parasites are ingested; parasites are then released and invade and multiply in the GI tract; the parasitic cell ruptures host and disseminates throughout the body.

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