Understanding Infection Chapter 2

 An infection is a response to a pathogen, or disease-causing substance, that enters the host’s body.  It results when tissue-destroying microorganisms enter and multiply in the body.  Some infections take the form of minor illness  Others result in a life-threatening condition called sepsis, which causes widespread vasodilation and multiple-organ-dysfunction syndrome (MODS).  Infection-causing microbes  Four types of microorganisms can enter the body and cause infection: viruses, bacteria, fungi, and parasites.

Infection-causing microbes  Viruses, are microscopic genetic parasites that may contain genetic material, such as DNA or RNA. They have no metabolic capability and need a host cell to replicate.  Viral hide-and-seek  Viral infections occur when normal inflammatory and immune responses fail.  Some viruses surround the host cell and preserve it; others kill the host cell on contact.

Bacteria, are mono cells that have no true nucleus and reproduce by cell division. Pathogenic bacteria contain cell-damaging proteins that cause infection, which may be: exotoxins—released during cell growth (cause fever and aren’t affected by antibiotics) endotoxins—released when the bacterial cell wall decomposes; classified by their shape, growth requirements, motility, and whether they’re aerobic or anaerobic

Infection-causing microbes  Fungi  Fungi are nonphotosynthetic microorganisms that reproduce asexually (by division).  They’re large and contain a true nucleus.  Fungi are classified as: yeasts—round, single-celled, anaerobes molds—filament-like, multinucleated, aerobic.  There’s a fungus among us  Although fungi are part of the human body’s normal flora, they can overproduce, especially when the normal flora is compromised.  vaginal yeast infections can occur with antibiotic treatment because normal flora are killed by the antibiotic, allowing yeast to reproduce.

Infection-causing microbes  Infections caused by fungi are called mycotic infections because pathogenic fungi release mycotoxin.  Most of these infections are mild unless they become systemic or the patient’s immune system is compromised.  Parasites  Parasitic infections are uncommon except in hot, moist climates.  Parasites are single-celled or multicelled organisms that depend on a host for food and a protective environment. Most common parasitic infections, such as tapeworm and tick infestations, occur in the intestines.

Barriers to infection  The body has many built-in infection barriers:  the skin and  secretions from the eyes, nasal passages, prostate gland, testicles, stomach, and vagina. Most of these secretions contain lysozymes. Other body structures, such as cilia in the pulmonary airways that sweep foreign material from the breathing passages, also offer infection protection.

Trillions of harmless inhabitants  Normal flora are harmless MOs that reside on and in the body.  They’re found on the skin and in the nose, mouth, pharynx, distal intestine, colon, distal urethra, and vagina.  Many of these MOs provide useful, protective functions ( intestinal flora help synthesize vitamin K, which is important for blood- clotting)

The infection process  Infection occurs when the body’s defense mechanisms break down or when certain properties of MO (virulence)or toxin production, override the defense system.  Other factors that create a climate for infection include:  poor nutrition stress humidity  poor sanitation  crowded living conditions pollution dust medications  hospitalization (Nosocomial infection).

Enter, attach, and spread  Infection results when a pathogen enters the body through:  direct contact,  inhalation,  ingestion, or  an insect or animal bite. The pathogen then attaches itself to a cell and releases enzymes that destroy the cell’s protective membrane.  Next, it spreads through the bloodstream and lymphatic system, finally multiplying and causing infection in the target tissue or organ.

Striking while there’s opportunity  Opportunistic infections; infections that strike people with altered, weak immune systems.  For example, patients with acquired immunodeficiency syndrome (AIDS) are plagued by opportunistic infections such as Pneumocystis carinii pneumonia