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.
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.
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.