INFECTION AND INFECTION PREVENTION
Among the various causes of disease, infectious agents gain importance in the clinic, as they can be transmitted from person to person.
Infectious agents are can be grouped into the fol-lowing categories. Arranged in order of structural complexity, they include prions, viruses, rickettsiae, chlamydiae, bacteria, fungi, algae, protozoa, meta-zoa, and Insecta.
Prions are proteins that do not have genetic mate-rial. However, they are infectious and capable of du-plication. Some diseases associated with prions are Creutzfeld-Jakob disease, kuru, scrapie and, in ani-mals, mad cow disease (bovine spongiform en-cephalopathy)
Viruses are the smallest of these agents. They con-tain DNA or RNA strands enclosed in a protein coat and require a living cell for replication and survival. On entering a host cell, the virus directs the nucleus of the cell to function differently and enable viral replication. New viral particles are formed in the host cell and these particles are liberated into the extra-cellular fluid to infect more cells.
Rickettsiae, chlamydiae, and bacteria are simplecells that are small (about 1 micron), with the DNA material enclosed in a cell membrane. They lack a nu-clear membrane. These organisms need specific envi-ronments for survival. The rickettsiae and chlamy-diae, similar to viruses, mainly depend on host cells for survival. Some bacteria are aerobic organisms that require oxygen for energy; others are anaerobic organisms that can survive without oxygen.
Algae, protozoa, and fungi are microorganismsthat have membrane-bound organelles and a nucleus. Algae are organisms that produce oxygen as a prod-uct of photosynthesis. Protozoa are unicellular or-ganisms that may contain flagella. They have a more complex life cycle. Fungi are microorganisms that grow as a mass of branching, interlacing filaments and include molds. Yeasts, which are also forms of fungi, do not have a branched appearance.
Metazoa and Insecta are multicellular parasitesthat affect humans. Metazoa include worms and flukes, and Insecta includes ticks, fleas, and Sarcoptesscabiei (which causes scabies) that transmit or causedisease.
The ability of the infectious agent to cause disease is called pathogenicity. Organisms that readily cause disease are said to be virulent. Organisms that cause disease only when the immunity in a host is low are called opportunistic pathogens. Knowledge of the characteristics of the various organisms helps hu-mans construct strategies to keep infections at bay.