Posted by Tara Smith on July 28, 2005 11:42 AM

Anyone who’s been involved in E/C debates has likely heard more than once that “evolution is a religion” (see, for instance, Matt Young’s thread here ). Some opponents suggest that because the theory has been modified somewhat over the years since Darwin’s original proposal, it’s a “theory in crisis,” or assert some other prediction of its imminent demise. Others have stated—correctly, in my opinion—that evolution has as much or more support as the germ theory of disease. So why do people attack evolution, but not the germ theory? Let’s compare the two.

First, a bit of history (and I do mean “a bit;” keep in mind I’m leaving a lot of people, events, and ideas out here). Even in the 19th century, the idea that some form of live contagion served to spread disease was not new. Indeed, the “germ theory” of disease has been around in a raw form for centuries. One of the earliest references to this theory appears in On Agriculture by Marcus Varro in 36 BCE, warning about building near swampy land “…because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases.”

Variations on this theme remained for centuries, but by the 1800s, the main competitor to the idea of some kind of “minute creatures” caused disease was the “miasma theory,” which held that miasmas—noxious atmospheres produced mainly by decaying organic matter—produced disease. This was the dominant theory, and the idea of germs as a root of disease did not gain a high level of support until put forth more formally by Henle in 1840:

The material of contagions is not only an organic but a living one and is indeed endowed with a life of its own, which is, in relation to the diseased body, a parasitic organism.

Though Henle formalized the germ theory of disease in the mid-1800s, there was resistance to the idea, and evidence was lacking. John Snow’s epidemiological studies of cholera were suggestive of an organism rather than a miasma, but Snow did not isolate the organism himself, which could have further cemented the connection.

In the 1860s, Pasteur demonstrated the existence of pathogenic organisms—bacteria and “filterable agents” (viruses) which could cause disease. In the 1880s, Henle’s student, Robert Koch, isolated the causative agents of both tuberculerosis and cholera. More importantly, Koch formalized a way to determine whether an infectious agent was a cause of disease; these have been modified over the years and are known as Koch’s postulates.

  1. The organism must be present in every case of the disease, but not in healthy individuals.
  2. The organism must be isolated from the host with the disease and grown in pure culture.
  3. The specific disease must be reproduced when a pure culture of the organism is inoculated into a healthy susceptible host.
  4. The organism must be recoverable from the experimentally infected host.

These postulates have been the “gold standard” for determining an infectious cause of disease for the past 125-odd years. However, even from the beginning, they were less than perfect. Using Koch’s postulates to determine the cause of hog cholera, a disease that killed more than 13% of the hogs in the U.S. in several outbreaks in the late 1800s, Daniel Salmon and Theobald Smith determined that the hog cholera bacillus (later named Salmonella cholerae-suis) was the cause of this disease. Koch and Pasteur agreed with them, and indeed, Salmon and Smith had fulfilled Koch’s postulates: they consistently saw the organism in hogs with hog cholera, they isolated it and grew it in pure culture, saw the disease when they infected healthy hogs, and recovered the bacterium. So, how could they be wrong? In 1903, Marion Dorset demonstrated that hog cholera is actually a viral disease, and that the virus had simply been present in the “pure” bacterial cultures. Oops. Thus, an early test of Koch’s postulates—a central principle of the germ theory of disease—failed miserably.

So, Koch and Pasteur were wrong in this case. Does an incorrect statement by the early advocates of an idea disprove the theory? Of course not: it stands or falls on its own merits, regardless of misapplication or incorrect conclusions drawn by using Koch’s postulates. However, there are certainly weaknesses in the theory that merit a second look. I’ll take the postulates one by one:

The organism must be present in every case of the disease, but not in healthy individuals.

Here we run into the problem of the carrier state and sub-clinical infections. A person may show no signs of disease, but may carry the organism on or in their body. Additionally, a disease may be caused by a ubiquitous agent (such as Epstein-Barr virus, which infects ~95% of the population) that causes serious illness relatively rarely. In both cases, the presence of the organism in healthy people makes it more difficult to definitively associate it with a particular disease. Additionally, a disease may develop only after the organism which caused it has been eliminated from the body. Therefore, attempts to culture it will come back negative, resulting in a misleading conclusion that the organism under investigation does not cause the disease in question.

The organism must be isolated from the host with the disease and grown in pure culture.

Here we run into problems of isolation and culture. Not all organisms can be grown in pure culture in the laboratory. Indeed, as I mentioned here, it is estimated that only ~150 out of roughly 400 oral bacteria have ever been isolated; one can imagine how many species of pathogens remain unknown to us in the environment.

The specific disease must be reproduced when a pure culture of the organism is inoculated into a healthy susceptible host.

Here we come to the problem with models. What is a “healthy, susceptible host?” Certainly we can’t go around giving other humans deadly disease, so we most often rely on animal models to replicate the course of infection. However, not all diseases have good animal models, and some pathogens will infect no other species but humans. Additionally, some diseases are not due to an immediate effect of the pathogenic organism, but are due to more distal effects of the organism. For example, the response of the host’s immune system to certain strains of Streptococcus pyogenes can result in rheumatic fever or rheumatic heart disease months or years after the initial infection, and long after the bacterium is cleared from the body. Additionally, this post-infectious outcome seems to occur more frequently in individuals of a certain genotype; so the pathogen genetics plus the host genetics plus the environment (including the presence of other pathogens) all must be taken into consideration when examining development of disease.

The organism must be recoverable from the experimentally infected host.

Assuming we were able to isolate a pure culture in the first place, and have a decent model of infection, re-isolation still isn’t always possible, as many of the problems with initial isolation are also present upon re-isolation. Additionally, the organism may have been cleared by the immune system, or may be present in one organ but not another (for example, the spleen but not the blood, so where samples are taken from may make a difference).

Looking at all of these problems, it might seem that the germ theory of disease isn’t quite as straightforward as many people assume it is. And it’s not—but there are ways to overcome the limitations. Rather than using pure culture methods, we can use molecular diagnostics (such as PCR) to determine the presence of various organisms. Rather than use solely animal models, we can use serological evidence or epidemiological studies to determine a correlation or investigate a cause/effect relationship between an organism and a specific disease in the human population. We can even use these types of studies to investigate infectious causes of chronic diseases, where the infectious exposure may be decades removed from disease development. Yet I’ve not seen many challenges to the “naturalistic” assumptions underlying the GToD as I do with challenges to evolution, nor have I heard the claim that the germ theory is a “religion,” as anti-evolutionists claim about the theory of evolution. Even the religious connections are there; certainly the Bible has been used by some to support the idea that disease is a punishment from God:

So now the LORD is about to strike you, your people, your children, your wives, and all that is yours with a heavy blow. You yourself will be stricken with a severe intestinal disease until it causes your bowels to come out.” Then the LORD stirred up the Philistines and the Arabs, who lived near the Ethiopians, to attack Jehoram. They marched against Judah, broke down its defenses, and carried away everything of value in the royal palace, including his sons and his wives. Only his youngest son, Ahaziah, was spared. It was after this that the LORD struck Jehoram with the severe intestinal disease. In the course of time, at the end of two years, the disease caused his bowels to come out, and he died in agony. His people did not build a great fire to honor him at his funeral as they had done for his ancestors.

(2 Chronicles 21:12-19)

The LORD will strike you with wasting disease, fever, and inflammation, with scorching heat and drought, and with blight and mildew. These devastations will pursue you until you die. The skies above will be as unyielding as bronze, and the earth beneath will be as hard as iron. The LORD will turn your rain into sand and dust, and it will pour down from the sky until you are destroyed. “The LORD will cause you to be defeated by your enemies. You will attack your enemies from one direction, but you will scatter from them in seven! You will be an object of horror to all the kingdoms of the earth. Your dead bodies will be food for the birds and wild animals, and no one will be there to chase them away. “The LORD will afflict you with the boils of Egypt and with tumors, scurvy, and the itch, from which you cannot be cured. The LORD will strike you with madness, blindness, and panic. You will grope around in broad daylight, just like a blind person groping in the darkness, and you will not succeed at anything you do. You will be oppressed and robbed continually, and no one will come to save you.

(Deuteronomy 28:20-29)

I will heap calamities upon them and spend my arrows against them. I will send wasting famine against them, consuming pestilence and deadly plague; I will send against them the fangs of wild beasts, the venom of vipers that glide in the dust. In the street the sword will make them childless; in their homes terror will reign. Young men and young women will perish, infants and gray-haired men.

(Deuteronomy 32:23-25)

Once upon a time, a prevailing view was that God sent plagues as a punishment for sin, and early in the days of the germ theory (like evolutionary theory), some were unsure how it could be reconciled with Holy Scripture. Indeed, there was resistance to the thought of these “germs” as a cause of disease at all, as it took away the aspect of morality that was previously linked to manifestation of many diseases. Disease was often thought to be due to moral failings, and specifically, excesses: too much anger, jealousy, gluttony, or sex, either in an individual, or in the population. Yet as the evidence for the germ theory piled up, religious views were modified to coincide with science: perhaps God was acting through nature, sending these pathogenic organisms instead of causing disease directly. This idea rears itself over and over, and was commonly cited even in the beginning of the HIV epidemic (often referred to as a “gay plague” in the early 1980s, and seen as evidence of God’s wrath). Still, very few people have a problem accepting a naturalistic cause for infectious disease (even if they believe God instigated it).

There are still tons of unanswered questions in the field of infectious disease, and as we learn more, we find it’s not always as simple and straightforward as Koch’s postulates suggest. But this is not a reason to be rid of them, nor to doubt explanatory power of the germ theory of disease. Koch’s postulates are still the gold standard for determining disease causation, and have served us remarkably well through the years. To dismiss the germ theory of disease as a “faulty model” because it’s often more complicated than a simple “one germ–>one disease” correlation is beyond naive, and demonstrates a lack of understanding of the process scientists go through when investigating these issues. And to hold evolution up to a higher standard than the germ theory, or to call it a “religion” but give the germ theory a free pass, when both have many areas of intense and exciting research devoted to filling in gaps in our knowledge, makes no sense to me.