1 About Infectious Disease

Reading 1

Introduction to About Infectious Disease

Have students read the introduction to the module. The purpose of this reading is to provide students with an historical perspective on the discovery of microorganisms, provide with them with a short introduction to microorganisms and inform them of what they will be exploring in the module.

Read this to get some background on module 1.

I. Discovering the microbial world

In 1674 Anton van Leeuwenhoek peered into a drop of pond scum through a lens mounted between two metal plates (see Figure 1) and discovered an incredible world of microscopic organisms that was totally unknown to anyone at that time. In this drop of water van Leeuwenhoek observed a vast diversity of tiny “animalicule” and “cavorting beasties” that were “so exceedingly small that millions of millions might be contained in a single drop of water”.

Figure 1: Van Leeuwenhoek's Early Microscope

Using an array of handcrafted microscopes that could magnify up to 300X, he went on to observe and describe protozoans such as Vorticella and Volvox, tiny animals called rotifers, spiral, grape, and rod-shaped bacteria, and parasites such as Giardia, sheep liver flukes, and trypanosomes (see Figure 2).

Figure 2: Van Leeuwenhoek's Animalicules
Courtesy of Wikimedia Commons

Since van Leeuwenhoek’s discovery of this parallel universe of microscopic organisms, an enormous amount has been learned about microbes and their profound effects on all living things.

Thousands of different types of viruses, bacteria, and parasites have been identified and their different modes of sustaining life characterized. There are doubtless more to be discovered.

II. Life in the microbial world

Essential to the integrity of soil, water, air, and all living things, microbes make life possible by carrying out vital processes and functions. In recent years scientists have been investigating the human microbiome, that population of more than 100 trillion viruses, bacteria, and parasites that call the human body home. Bacteria alone constitute about 2 to 6 pounds of a person’s body weight but outnumber human cells 10 to 1, an indication of their minute size. (See Figure 3). Human skin alone harbors more than 100,000 microbes per square centimeter: no matter how hard you scrub with antibacterial soap you can’t get rid of them all.

size comparison
Figure 3: Size comparison of a human liver cell, a bacterium, and several types of viruses. Click to view a larger version of the image.
Courtesy of Biology-forums.com

Many of these microbes digest food, combat disease-causing bacteria, and synthesize essential nutrients and vitamins. Others are just along for the ride, peacefully coexisting in commensal harmony with the cells in your body.

Because of their much smaller size, it is unlikely that van Leeuwenhoek observed any viruses in his droplet of pond scum (See Figure 3). In the late 1800’s a Russian biologist, Dimitri Ivanovsky determined that something much smaller than bacteria was causing a disease that was destroying the tobacco crop in Russia. In 1935 an American scientist, Wendell Stanley, identified this microorganism as tobacco mosaic virus. Since then, many hundreds of virus types have been identified and characterized. Recently, a blood test is in development that might possibly provide a complete “viral history”, identifying all the viruses that have ever infected a person, some which have made their presence known through symptoms of the common cold, measles, and flu, and others, which caused no disease symptoms at all.

While most microorganisms are beneficial or at least neutral in their impact, a subset exists in the dark side of the microbial world that cause debilitating and sometimes fatal infections in plants and animals. The following module focuses on the inhabitants of this dark side, the causative agents of infectious disease. You will learn about epidemics of the past, the nature of infectious diseases and the culprits that cause them. You will then explore in depth two viruses causing serious concerns globally, Ebola and measles, and begin learning about COVID-19.