You go for a swim, and you don't even notice the tiny worm that burrows into your skin. It slips into a vein and surges along through the blood for a while. Eventually it leaves your blood vessels and starts creeping up your spinal cord. Creep creep creep, it goes, until it reaches your head. It curls up on the surface of your brain, forming a hard cyst. But it is not alone%u2013every time you've gone for swim, worms have slithered into you, and now there are thousands of cysts peppering your brain.
And they are all making drugs that are seeping into your neurons. These drugs are a bit like Prozac, except far more sophisticated. They target certain neurons in certain parts of the brain, altering your behavior surgically, without unwanted side effects.
You don't know what's happening to you. But in situations in which you'd expect to feel scared or stressed, you just want to race around. You whirl in circles, doing whatever is necessary to get the attention of the very thing that terrifies you. Thanks to your uncontrollable flailing, that terror finds you, and you are destroyed.
This is how I imagine you'd feel if you were a fish infected by a parasitic worm called Euhaplorchis californiensis.