Lecture 11 evolution continued, and the origins of new epidemics. Resistant genes act against similar toxins (e. g. pump out organochlorides) If similar toxins used, pests are already partly resistant, resistance builds rapidly (polygenic process) Must switch to unrelated chemical (new toxins now very expensive to find) Similarly, drugs reduce helpful bacteria in our guts - resistant bacteria may grow faster. Most harmful human parasites are symbionts of other animals (zoonosis) Evolutionary pressure is less harmful on commensals. Commensal: symbiont that benefits and does not harm the other. Parasite: symbiont that benefits and harms the other partner. A parasite is not transferred if the host is killed rapidly. The parasite benefits if the host is abundant. Health = natural balance disease is often a change in this balance, not colonisation by new bacteria. Use some food in our gut (almost harmless) Partially digest food, eat harmful excretions of bacteria.