Infectious Diseases
Overview
Many types of organisms can cause disease in humans. They are referred to collectively as pathogenic organisms (patho- from the Greek for 'disease' and -genic meaning 'producing'). The smallest are viruses, then there are microbes such as bacteria, fungi, and protozoa. There are also larger multicellular animals that are parasites on humans or can inflict tissue damage.
Viruses
These are tiny particles that consist of a DNA or RNA core surrounded by a protective coat, usually made of protein. They insert their own genetic instructions into the genome of the host cell, causing the cell to produce multiple copies of the virus. The new viruses leave the host cell to infect other cells. Viruses generally damage or kill the host cells, although this may not become apparent immediately - 'slow viruses' have an eftect after many years.
Bacteria
A bacterium is a small type of cell (prokaryote) in which the genetic material is not enclosed in a nucleus but floats freely within the cytoplasm. Some bacteria secrete toxic substances that damage human tissues, while others become parasites within cells. Some bacteria form colonies within the body and disrupt normal function. Bacteria may be classified according to several criteria, for example physiology (aerobic or anaerobic), staining properties (Gram-positive or Gram-negative), and shape and size (bacilli, cocci, curved or spiral rods, 'large' or 'small').
Fungi
Fungi are relatively simple organisms that resemble plants but are without chlorophyll. They are classified as yeasts (single-celled) or moulds (multicellular). Most pathogenic fungi parasitise tissues at or near a body surface, such as the skin or mucous membranes. A few fungi can become distributed throughout the body.
Protozoa
Protozoa are single-celled organisms, each with a distinct nucleus (unlike bacteria), and are therefore classified as eukaryotes. Protozoa can infest body fluids and cells, and include amoebae, flagellates, ciliates, and sporozoa.
Pathogenic Animals
Large multicellular animals that produce disease or injury include nematodes (roundworms), platyhelminths (flatworms and flukes), arthropods (mites, ticks, lice, wasps, bees, mosquitos, and spiders), and snakes.
Responses of the body to infection
Infection is a fact of life. Most interactions between humans and infective organisms go unnoticed -the body's defence mechanisms are able to prevent the organism from proliferating in or on the body. Only occasionally is the outcome an infectious disease.
An infectious organism must be able to adhere to, colonize, or invade the host and proliferate at least to some extent. As a result, animals have evolved elaborate defence mechanisms at their various interfaces with the environment. Defensive barriers include the skin (mechanical protection, decontamination by shedding of outer flakes and addition of glandular secretions, and maintenance of normal flora), lining of mouth and pharynx (mechanical barrier, decontamination by saliva, lymphoid tissue such as tonsils, and normal flora), gastro-intestinal tract (acidity of gastric environment, peristaltic movements, mucus, antibodies in intestinal secretions, lymphoid tissue, presence of normal flora and fauna in large intestine), airways (mucus secretions moved by ciliated lining, antibodies in secretions, macrophages), urinary tract (mechanical barrier, movement of urine), conjunctiva of eye (mechanical, flow of tears containing bacteriostatic substances), and vagina (mechanical, normal flora). If an infectious agent manages to penetrate the defensive barriers and enter the body tissues, the body reacts with an inflammatory response and a variety of immune responses.