Magical numbers

Abstract

A number is a mathematical construct used in counting and is really three things in one: the notational symbol (5), the written number (five) and the abstract concept that we can count the number of digits on one hand. No wonder some children struggle to become numerate. The Book of Numbers is the fourth book of the Old Testament and gets its name from one of the first recorded censuses. Moses, expelled from Egypt and wandering in the wilderness of Sinai with the Israelites, was ordered by God to record the number of men 20 years old and upward able to bear arms. The census showed that over 600 000 Israeli men were fit for military service.1 Moses and the Israelites spent 40 years in the wilderness. But, I digress. As an infectious disease physician, I have always been impressed by doctors’ mystical belief in the necessity that a course of antibiotics, once started, must be completed. This dogma is based on the dubious theory that antibiotic resistance is more likely if antibiotics are stopped early than if the ‘full course’ is completed. Since antibiotic resistance is a nice example of Darwinian natural selection in action,2 it seems logical that selection of a resistant strain will be favoured by long rather than short courses of antibiotics. But, we are in the realms of mythology, not logic. Another striking thing about antibiotic courses is that their duration is often without strong scientific evidence.3 Nevertheless, the recommended duration is almost always 5 or 7 days, or a multiple of one of these. No one prescribes 4 days or 6 days, or a Beatles-like 8 days a week of antibiotics. Clearly, we are using magical numbers. Wherein lies their magic? Five is the number of digits on a human hand or foot; indeed, almost any mammal, reptile or amphibian with fingers or toes has five on each extremity. The natural basis of five is reinforced by the five senses: hearing, seeing, smelling, tasting and touching. From a religious perspective, Jesus Christ traditionally has five wounds, there were five books of Moses, Muslims pray to Allah five times a day, there are five sacred Sikh symbols (called the five Ks because they all start with K), and Greek philosophers like Aristotle believed that there were five elements to the universe: water, fire, earth, wood and metal. I could continue to the five-pointed religious star (the pentagram), the five-sided pentagon or the five lines to a musical stave (Every Good Boy Deserves Favour), but I think I have made my point that the number five has both religious and natural import. The number seven is the days of Creation. There are seven virtues and seven deadly sins. The number seven recurs throughout the Bible. In Islam, there are seven layers to the Earth and seven levels in heaven. There are seven colours in the rainbow. Humans and most mammals have seven cervical vertebrae. There is even a seven-spot ladybird. Again, the concurrence of religious and natural manifestations bestows mystical significance. There are seven notes in a musical scale. Rome was built on seven hills, and over 50 cities from Amman to Zevenbergen claim the same distinction. JK Rowling wrote seven Harry Potter books during the course of which the evil Voldemort splits his soul into seven dark magical objects called horcruxes, which Harry has to destroy to prevail. Incidentally, if you have not read Harry Potter or listened to Stephen Fry reading the books on audiotape, please do. You have no idea what you are missing. But, I have exceeded my word limit, and my number is up. For many infections, it is preferable to stop antibiotics once symptoms resolve and not complete a course whose duration is based on a magical number.