Some might say, sure, Al, everyone gets the cold, it's no big deal. And I'd agree. But this... this is no mere cold. It may have all the symptoms of the common virus, and it feels like it. But I know better: I know that there is something which has taken up residence in my head, and it means to torment and despoil my temperament.
Decription? One might as well ask to paint a picture of "irritation," or "subtle," or "mischief." It does not dwell in our paltry human dimensions, but in a veil between worlds, where it can exude its dripping malfeasance without physically occupying room in my brain: that makes it safe from prying foreceps and particularly violent sneezes. All the while wheezing and squeaking with unspeakable glee at the havoc It is causing.
But fear not: even as I type with my nose simultaneously blocked and running, even as a devilishly faint but maddening tickle in my throat conspires to elicit a coughing fit, even as my eyes stare bloodshot at the world, my forces are marshalling. Around the Lymph Node fortresses great armies are gathering. The fabled White Blood Cells, holy orders of warriors dedicated to driving out the heathen bacteria, the heretical toxin, and the insidiuous virues, have summoned the Immune System: the cunning Lymphocytes detect their foe, and the relentless Phagocytes consume and destroy them. Then a host which seems to encompass the membrane: the serried ranks of dendritic cells, the platelet clans, the bacterial foederati, the wild tribes of antibodies. Soon they shall march upwards to the battle, steel in hand, to expel the invaders...
(This extrapolation of the immune system brought to you by 8-year-old Aly, though by no means the only one) In other words, I have a cold, I'll get back to you, I really think someone could make a fun Sword-and-Sorcery story using the immune system as a setting. Educational and fun!