Parasites pose problems for the ark
In their efforts to take as much of the Bible literally as they can stand, the creationists maintain that not only is the fable of Noah and the ark inerrant and correct, but that The Flood explains much of the world around us. However, in order to do so, they must ignore innumerable serious problems with the global flood myth. One series of such problems is the survival of many parasites to the present day.
Parasites of the insect order Strepsiptera pose just such a problem. Strepsipterans are parasites related to the beetles, and as larvae, attack other insects. When the larvae hatch, they search for, then burrow into a potential host, feed inside of the host's abdomen, and eventually pupate. The free-living, adult males live for a few hours to a day after metamorphosis, while the female remains within the abdomen of the adult host, sterilizing or "stylopsizing" it. In particular, those strepsipterans which parasitize bees have complicated and haphazardous lifecycles which involve having the female lay eggs in flowers the current host is visiting, and having the larvae wait be transported back to the appropriate host's hive, and into the cells of bee larvae to attack. If, as creationists claim, a single pair of "bee kind" was on the ark, then bees would be extinct, as they would have to have been infected by the pair of "strepsipteran kind" and would have had their children eaten or starved to death, if they were able to have any at all. Yet bees continue to exist today, as do their strepsipteran parasites.
Numerous diseases are specific to humans, thus must have been infecting somebody on board the ark. Examples include African Eye Worm (Loa loa) and Wuchereria bancrofti (one cause of elephantiasis). Elephantiasis is a particularly debilitating disease, yet someone on the ark must have been infected by it. These diseases do not infect any animals other than humans. Yes, they may use insect vectors to travel between human hosts, but they cannot remain long in an insect with a short lifespan. There exists no other reservoir to store the parasites.
Which person on the ark was unlucky enough to have been nominated by God to carry all these horrible diseases? Did they share? Did one human carry all the different elephantiasis causing worms or were they spread around? Given how quickly some of these diseases spread, did everyone have them? How did Noah's crew do all the massive amount of work needed to keep the animals alive while infected with all these diseases? Creationists do not seem eager to investigate these questions, largely because any such research would only serve to highlight how foolish the entire claim is.
Viral parasites pose a special problem. Most viruses are very host specific and cannot survive long without a host. Because their hosts develop immunity to a virus after infection, viruses require a large population of hosts to infect. To see what I mean, imagine a world with only eight people in it. One carries smallpox. He gets the disease. It spreads to the other eight people. They all have smallpox. But now that everyone has had it and survived (or not), there is no one else to infect. Unable to live without a host, the smallpox virus goes extinct 4000 years earlier than it actually did. Nearly all viruses have this problem. They can only survive if the population is producing new hosts fast enough to keep the virus from extinction. If Noah's Ark were true, not only would nearly all human viruses be extinct, so would nearly all plant and animal viruses! This is clearly not the world we see today.
Confronted with these problems, most creationists simply beg off the problems, but a few have attempted to justify creationism in the face of the contrary evidence above.
The most common response is the typical creationist technique of highlighting exceptions as if they were the rule. Chickenpox, for example, might be brought up as a virus that could survive despite the reservoir problem. Chickenpox virus persists in its victims as an asymptomatic infection for the remainder of their lives, but may flare up if the immune system becomes compromised. This frequently happens in elder life and causes a condition knows as shingles. People who have not gotten chickenpox may contract it when exposed to someone experiencing shingles, thus circumventing the problem given above. Thus, viruses could, we're assured, survive the events of Genesis.
But the survival of a few exceptions does not explain the survival of the others. Chickenpox may persist in its hosts for decades, but smallpox does not. Neither does mumps, measles or any number of other viruses. The argument above is like saying, "Some people survive gunshot wounds, therefore nobody has ever died from being shot!"
Another claim, ironically enough, requires the creationists to embrace large amounts of evolution. If all of the other parasites evolved from the few survivors, they do not pose a serious problem.
There are numerous problems with this. The major problem, it requires preposterously fast rates of evolution. Influenza is a very rapidly evolving virus, but after centuries of watching new forms evolve and adapt, there hasn't been any major change to it's symptoms or style of infection. If the creationist claims were correct, we should be seeing major new viruses, worms, and bacteria emerge almost constantly! Instead, we know from historical records that the diseases we have have been relatively stable throughout human history.
Perhaps worse for creationism is that such high rates and degrees of evolution severely strain creationist notions of what evolution is supposed to be able to accomplish. Humans experience a tremendous diversity of parasites, if they all evolved from a realistic load that could be carried in eight humans on the ark, this means that "created kinds" are tremendously broad, far broader than most creationists would be comfortable with; certainly broad enough to make the "gap" between humans and chimpanzees look small.
It is also claimed that many parasites are worse now than they were at the time of the flood. Therefore, the crew of the ark could have been infected with all manner of parasitic organism without being seriously debilitated. This ties into creationist notions of The Curse, where life has gotten worse since The Fall. However, it flies in the face of established biology. For one thing, fossils (human and otherwise) show parasitic diseases every bit as bad as modern ones. For another, history shows us that most diseases adapt to become less and less harmful to their host, not worse. There are some exceptions, but these are understood and do not justify the creationist claim. (It also contradicts numerous creationist arguments, such as Neanderthal Man, found in France over 50 years ago, is an old man who suffered from arthritis, Neanderthal was based on a disfigured human, and Neanderthals were humans with rickets.)
Creationists have no good explanation for the diversity of modern parasites. They cannot explain how so many different types survived the flood nor can they explain how they evolved afterwards without contradicting plain fact. Parasite survival remains one of many thorny problems that reveal the Ark story as just that: A story, nothing more.