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Hundreds of serious diseases and illnesses are caused by various viruses, bacteria, and parasites. Many of these pathogens cannot survive more than a few days outside a human host. Among these are measles, smallpox, typhus, rubella, yellow fever, hepatitis, Ebola, and numerous sexually transmitted diseases, including AIDS, gonorrhea, chlamydia, HPV, trichomoniasis, and syphilis. Human specific parasites include various protozoans, fungi, yeasts, insects, and helminthes (parasitic worms). There are scores of human-infesting round worms alone, including roundworms, hookworms, whipworms and pinworms, all of which require human hosts to complete their life cycles. They produce symptoms ranging from constipation and stomach bloating to anemia, blindness, asthma, diarrhea, fatigue, joint pain, limb swelling, severe itching and rashes, and malnutrition.
Young earth creationists, who believe in a the Biblical Flood was a literal, global deluge which occurred only a few thousand years ago, face a serious dilemma. They must explain how all these diseases survived the year-long Flood unless most infected Noah and his family (eight individuals). But if this was the case, how did the Noah and his family, and thus the human race, survive? A similar problem exists regarding the hundreds of diseases that infect specific many animal species, namely: how did each disease survive unless many ark species were infected with many such diseases, but if the animals were so infected, how did the animals survive? This question is even more accute considering these ill animals would have had to survive the violent ark voyage itself, then debark into a foreign environment very different from their original home.
Young-earth creationists (YECs) Carl Wieland (1994) and Jonathan Sarfati (1997) try to address these problems with a number of proposals, none of which go very far toward a plausible solution, causing Wieland to acknowledge that the problem is a "challenge" for creationists. One proposal was that some pathogens may have been less specialized in the past, and could have been carried by various animals on the Ark. Even today some microbes can infect both human and animal hosts, while many others cannot. However, if there were many more of the former before the Flood, it would at best exacerbate the problems noted above concerning the animals' survival, since only two of each "kind" of animal (or at most seven pairs, for "unclean" kinds) were aboard the Ark. Furthermore, it's implausible that all of the diseases that are human-specific today were animal diseases only a few thousand years ago (when YECs believe the Flood occurred).
Sarfati suggests that perhaps Noah and his family were hardier than modern humans, and that loss of disease resistance "is consistent with the general degeneration of life since the Fall." However, Noah was many generations removed from the Fall, and no matter how hardy he and his family were, they hardly could have escaped serious effects from simultaneous infections of numerous serious or lethal pathogens. Even one or two such infections often cause symptoms ranging from serious debilitation to death. Wieland suggests "evolutionists" exaggerate when they depict Noah staggering off the ark while infected with dozens of diseases. However, if any scientists drew such a mental picture, it would actually be kind toward YECism, since instead of staggering off the ark while so infected, Noah and his family would have expired long before it landed.
Sarfati and Wieland also propose that various pathogens might survive in insects or corpses, or in dried or frozen states. Wieland suggests the later might occur at the poles, but this contradicts the YEC tenet that the Flood waters covered the entire globe. He further suggests that dry conditions might occur inside floating vegetation mats. However, after forty days of continuous rain and the raging flood waters, it's hard to imagine any dry spots. Indeed, any such mats would have formed from water-drenched vegetation. The proposal also does not explain why scores of human pathogens would collect at any such dry locations. Even if they did, they would have to quickly infect Noah and his family after the flood, and somehow not kill or incapacitate them--bringing us back to the original dilemma. Besides all this, even when dry or frozen, many human-disease causing pathogens do not survive in nature more than a few days without a live host.
Likewise, diseases and parasites that survive in insect hosts or corpses generally only do so for short periods of time--usually far shorter than a year--no matter what the conditions. Many depend on living human hosts for survival. Wieland and others note that some pathogens like polio don't always show symptoms in their carriers. While true, the problem remains that scores of other diseases do, and many others are almost always very debilitating or deadly. YECs not only must explain how some survived the Flood, but how all of them did, including many STDs which require human hosts. This raises the question of how Noah and his family would have become infected with dozens of diseases (including many STDs) in the first place, especially since they were declared the only righteous people on earth. As far as I know, no YECs have ventured an answer to that. If Noah and his family had been infected with even a fraction of such parasites and other serious diseases, it's implausible that they could have survived at all, let alone managed to feed and care for tens of thousands of animals (many of whom would also have been sick and dying) on a violently tossing Ark for over a year.
Of course, YECs can propose that God miraculously protected the ark passengers from the effects of all these diseases. However, inventing such ad hoc extra-Biblical miracles (Isaac, 1998) seems to invalidate YEC claims that a global Flood is scientifically sound and consistent.
Mark Isaak (1998) notes that many host-specific diseases which don't kill their host are usually soon eliminated by the host's immune system. This begs yet another question: while using a Flood to purge the world of sinful people (drowning billions of innocent animals and many human children in the process), why wouldn't God simply let the pathogens go extinct, either by the above mechanism or the scarcity of human hosts. It seems ironic that many YECs would say that God would never allow a process like evolution because it involves death (even though physical death is a necessary part of all ecological cycles), and then turn around and suggest that perhaps during the Flood God used miracles to preserve scores of insidious and expendable pathogens, which are responsible for the suffering and death of millions of people (including many children) every year. Indeed, "Intelligent Design" advocate Michael Behe goes a step further. In his 2007 book "Edge of Evolution" he suggests that the parasite which causes Malaria was specifically designed, and later became resistant to the drug chloroquine through the further intervention of the "designer." In other words, Behe's envisions a designer (by which he means God, judging from his other writings), not only "designing" a deadly disease, but deliberately thwarting human efforts to eradicate it, so that millions more adults and children could suffer and die from it every year. In view of such reasoning, I suppose the idea of God intervening to save microbes during the Flood is not a stretch, but I doubt most sensible people would find such notions ethically or theologically sound, even if they were scientifically credible, which they are not (Levin, 2007).
Scores of serious and fatal diseases and illnesses were unlikely to survive a global Flood outside of human hosts, yet Noah and his family could not have survived if they were infected with them or even a significant number of them. Explaining how the family became so infected creates even more scientific and theological obstacles for any YEC/FG model. The simplest and most logical solution, which accords with mountains of other geologic and paleontological evidence, is that there was no recent global Flood. As many scholars allow in the face of such evidence, the Biblical Flood could have been regional, or the account largely allegorical.
Isaac, Mark, 1998, Problems with a Global Flood (2nd Edition) at: http://www.talkorigins.org/faqs/faq-noahs-ark.html#survival
Jonathan Sarfati, 1997. How did all the animals fit on Noah's Ark? frame left First published: Creation 19(2):16-19. March 1997. Web version a: http://www.answersingenesis.org/creation/v19/i2/animals.asp
Levin, David E., 2007, Reports of the National Center for Science Education, Issue 12, Jan.-Apr, pp. 38-40.
Wieiland, Carl, 1994. Diseases on the Ark. Creation Technical Journal. Vol. 8, No. 1, pp. 16-18. On line version at: http://creation.com/diseases-on-the-ark