It's apparently the death-knell for all theists: Parasites! Grotesque, painful, life-destroying Parasites!
Take this famous discussion of it by Stephen Fry. Parasites are proof positive that a loving God does not exist.
I was part of a debate on Thursday discussing "Is God worthy of worship?" One of our opponents, crowd-sourcing his material from eager Twitter followers, spent his talk listing some of nature's ugliest monstrosities. Horrific diseases and deformities were rattled off in quick succession. At points he played it for laughs, and he got them.
Which ought to make us think. If this is really being raised as the "problem of evil" why are comfortable westerners, sipping red wine in an Oxford College, sniggering about such horrors? Is this stuff really evil? In which case let's treat it seriously as a challenge to belief in a good God, recognising that all of us face such wickedness. Or is it not really evil? Is it just a freak-show, an object of macabre fascination, or—God forbid—an exercise in apologetic points-scoring? If it isn't actually evil, perhaps the lesson we should learn is 'Abandon all hope and adjust your expectations accordingly.' Well, ok. But a) drop the secret (or not so secret) glee regarding creation's monstrosities, b) realise you've solved the problem of evil but only by losing the right to call it "evil" and c) brace yourself for a much harder intellectual problem: the problem of good (of which, more shortly).
In all this, the greatest mis-step in the parasites conversation is to ignore (often times wilfully) the doctrine of the fall. To imagine for a moment that we can simply read God from creation is to engage in the kind of paganism roundly condemned in Scripture. As Francis Spufford says in Unapologetic:
To anyone inclined to think that nature is God, nature replies: Have a cup of pus, Mystic Boy.
The world is fallen. It is corrupted, cursed, 'knocked off its axis', disconnected from its true Life-source. To speak of parasites in the world does not put the merest dint in the Christian world-view. It only supports it.
Think about parasites. We're dealing with creatures that are, well, parasitic. In fact 5 minutes' meditation on parasites will pretty much give you the Christian doctrine of creation and fall.
These things cause monstrous perversions, hellish corruptions, wicked deviations from what should be. The disease and death they bring is not Right, it's wrong. This is not Light, it's Darkness. There is an original and ultimate life-giving source. And there is a secondary distortion which takes life.
This is the Christian doctrine of creation and fall: an original good perverted into corruption and death. Good is ultimate, Evil comes later to steal, kill and destroy. The Light is ultimate, the Darkness is a privation of the Light. First there is a straight line from which all crooked lines are corruptions.
But here’s the thing: to judge a line “crooked”, what exactly is “straight”? And if you want to avoid the conclusion that there is an Original Straightness to things, you might say “Ok, these lines aren’t definitively crooked, it’s just that everything’s messy.” Well ok, fine, but at that point you’re not wrestling with the problem of evil any more. You’re just saying “Things are messy. Stuff happens.”
And then you have to face a much greater intellectual hurdle: the problem of good. You see evil, as a secondary corruption of good, is not intellectually difficult to understand. (It's horrifically unpleasant and evokes understandably emotive reactions, but intellectually it's origins are understandable). On the other hand, Good—if it's not original and ultimate—becomes extremely difficult to explain. This is because Good and Evil are not symmetrically opposite to each other. They are like light and darkness: Light can illuminate darkness, darkness cannot darken light. Darkness is the absence or obscuring of Light in a way that is not true the other way around.
I'm speaking of light and darkness figuratively here, but they are powerful illustrations. When the Christian is asked "Where does the darkness come from?" They answer: "From a turning away from the light." When the atheist is asked "Where does the light come from?" the answer "From the darkness" seems absurdly improbable. If light does exist then it needs to be there from the beginning. But this is the Christian account of reality.
The atheists are right: parasites are powerful illustrations of the problem of evil. But they're also a perfect analogy for how evil works. It is derivative, privative, secondary. But once you've said that, you've essentially told the Christian story of the world. There's something Good and Life-giving and something came along to spoil it.
From Creation and Fall, the Christian can explain both good and evil. But if our "creation story" is effectively: "slime + struggle + selfishness" with no injection of an original Good, it's quite a stretch to end up with "selves, sentience and symphonies."
Parasites are horrible. As they work their way through the eye-ball of an 8 year old boy we are appalled. This is not simply painful, not simply ugly, not simply maladapted to life - it is wrong.
But let's also remember, parasites are parasites! There can't be parasites "all the way down". No, there is an ultimate and original Good by which to judge these things evil. And the Christian can hate this evil with a holy and almighty antipathy for we are seeing the work of God's enemy —an enemy Christ opposes with every drop of His own blood. We do not shrug our shoulders or snigger or adapt ourselves to the inevitable. We call evil evil and we fight it.