Understanding InfectionChapter 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