Infection And Response

A single bacterium settling into a warm cut can, in good conditions, split in two every twenty minutes. Thirty divisions later, before a working day is out, one cell has become more than a thousand million. Nothing about your body has changed in those hours except the number of uninvited organisms living inside it, and yet you may go from perfectly well to seriously ill. That is the arithmetic every immune system is built to beat.

This lesson follows that race from both ends. You will see how bacteria and viruses actually make an organism ill, meet the three jobs white blood cells do about it, and find out why recovering from a disease once often protects you for life. Then you will look at the two things people have added to those natural defences, vaccination and antibiotics, and at the uncomfortable reason the second of them is becoming less reliable every decade. Almost all of it is content both tiers sit; the small part that belongs to the Extension Tier alone is marked where it appears, so you always know what is being asked of you.