Know Your Enemy

Article excerpt

The world of infectious diseases and the agents that cause them is not so foreign if we take a little time to explore it.

Most of us will probably catch a cold this year. Someone in our family may come down with the flu as well. Those of us over 35 probably remember fevers, headaches, stomachaches, itching skin, and swollen glands as we suffered through the inevitable childhood diseases: mumps, measles, chicken pox, and perhaps others. We learned that by having the disease as children, we were building up immunity so we wouldn't have to suffer later.

Today, most American children are immune to those diseases. Thanks to childhood vaccinations, they have built up immunity without becoming ill. On the other hand, many common illnesses such as coughs and earaches afflict these children and cause sleepless nights for their parents. As treatment, children receive a course of antibiotic, prescribed for a period of days. However, when the symptoms fade in a few days, parental anxiety is relieved and the natural tendency is to stop giving the antibiotic. Thus parents and children treating a cough or ear infection, along with countless others who are prescribed an antibiotic course, unwittingly assist the microbial world to overcome our defensive weapons.

By discontinuing the treatment prematurely, we fail to kill all the microbes, leaving behind those survivors that are best able to withstand the antibiotic. Our priorities do not include fighting invisible and apparently harmless enemies. But the microbes are relentless. Their constant priority is to seek out advantageous living conditions and exploit available resources to achieve maximum growth and multiplication. Thus they turn every human lapse in treatments to a microbial advantage, adapting now just as they have been doing for hundreds of millions of years.

Deseases and disease agents

The roster of infectious diseases is imposing: measles, malaria, hepatitis (A, B, C, E, or G), sleeping sickness, cholera, leishmaniasis, AIDS, tuberculosis, dengue fever, and river blindness. The list could continue for many more lines. Some of the diseases, like AIDS, are new, while most are old. In the twentieth century, significant progress has been made in overcoming such diseases as measles, malaria, and tuberculosis. But in the last 20 years these three scourges have been occurring in increasing numbers.

Each infectious disease is caused by a particular microorganism, and their diversity is perhaps even more impressive than the variety of diseases themselves. In simplified terms, we may categorize disease agents according to their level of development of a cell-like structure. Thus, they can be acellular, unicellular, or multicellular, with sizes increasing correspondingly. Our survey of microbial disease agents will progress roughly from the smallest to the largest, covering viruses, bacteria, fungi, protozoa, and helminths (worms).

Viruses are acellular agents ranging in size from 20 to 200 millionths of a millimeter (nanometers). They consist essentially of a core of genetic material (RNA or DNA) surrounded by one or two layers of protein. Despite their small size, viruses exhibit a surprising variety of shapes and survival strategies, and they cause a comparable diversity of disease effects. Most viruses are so small that they could not be viewed until the electron microscope became available after World War II. Viruses are true parasites, requiring a host for replication. They cause diseases ranging from AIDS to smallpox, rabies, polio, measles, and the common cold. Measles is the second-largest cause of death by an infectious disease, even though an effective vaccine exists. Most of these deaths occur among children in India and Africa.

Single cells without a nucleus

The unicellular agents are divided into those that have no nucleus (the prokaryotes) and those that do (the eukaryotes). The multitudinous types of prokaryotes that cause human disease are encompassed in the large category of bacteria. …