Home > Articles

  • Print
  • + Share This
This chapter is from the book

Humans Live with Many Pathogens

MRSA is one type of pathogen, the collective word applied to microbes and viruses that cause disease. (The term microbe includes bacteria, some types of fungi, and protozoans.) Each type of microbe has a distinct lifestyle. Bacteria are single-celled organisms that reproduce by binary fission; each cell grows and then divides to form two new cells. Bacteria cause many of the diseases that make headlines: tuberculosis, flesh-eating disease, and anthrax. Pathogenic fungi include yeasts and molds. Yeasts are single-celled, whereas molds tend to grow as thread-like structures composed of many cells. (Some pathogenic fungi switch between the forms in response to the environment.) Yeasts and molds cause pneumonia, and in immuno-suppressed persons yeasts and molds can cause deadly systemic infections. Pathogenic protozoans, such as the types that cause malaria, are single-celled microbes that are often spread by insect bites. In tropical and subtropical regions, protozoan diseases are among the major killers of humans. Protozoa and helminths (worms) are usually called parasites rather than pathogens due to their larger size. In Antibiotic Resistance, we do not distinguish between pathogens and parasites, because antibiotics are used for maladies caused by parasites as well as by pathogens.

Viruses differ qualitatively from the cellular organisms just mentioned. Viruses cannot reproduce outside a host cell. They require the machinery of a living cell to make new parts. Indeed, one could argue that viruses are not alive even though they are composed of the same types of molecules found in microbes, plants, and animals. Another feature of viruses is that they are generally much smaller than microbes: An electron microscope is required to see most virus particles, whereas a light microscope is adequate for microbes.

Many microbes and viruses are found in and on our bodies (see Box 1-1). Some are beneficial; others are harmful. Some pathogens only occasionally cause infectious symptoms. For example, Mycobacterium tuberculosis enters a dormant state in most persons it infects, with a minority of infected persons exhibiting symptoms. However, immune deficiency enables M. tuberculosis to exit dormancy and cause disease. Other serious diseases arise from microbes, such as the yeast Candida albicans, that ordinarily live harmlessly in or on humans. This organism causes vaginitis with healthy women and more serious disease with immune-compromised patients.

Pathogens that normally grow only inside humans often have effective means of transmission. Mycobacterium tuberculosis and influenza virus are two that spread through air; Vibrio cholerae, the cause of cholera, contaminates drinking water; and many digestive tract pathogens move with contaminated food. (Salmonella typhi, the bacterium that causes typhoid fever, is an example.) Many other pathogens are spread by insects and ticks. Among these are the protozoans responsible for sleeping sickness and malaria, the bacteria that cause plague and typhus, and many types of viruses, such as the agent of yellow fever. Avoiding contact with pathogens is exceedingly difficult.

  • + Share This
  • 🔖 Save To Your Account