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Survival of the Sickest

Sharon Moalem

The Black Plague in the fourteenth century killed twice as many healthy males as females or children. Suggestion that the survivors were ones who had a mutation which locked up iron in their bodies and stopped virus getting it for food. So perhaps old doctor's remedy of blood letting had a reason behind it. After WW2 doctors in New Zealand gave Maori kids iron supplements because they were often aenemic but the injected ones turned out ot be 7 times more likely to die of infections

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When you drink alcohol, it gets converted to energy in several steps. An enzyme (alcohol dehydrogenose) coverts alcohol to another chemical (acetyldehyde). Another enzyme (acetyldehyde dehydrogenose) converts it to the chemical acetate, and another enzyme converts that to CO2, Fats and water. Many Asians have a genetic variation which produces a less active form of the second enzyme, so lots of acetylaldehyde doesn't get converted to acetate. The former is a lot more toxic and produces reactions like face flushing dizziness and nausea.

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Dark skin of Negroes protects folate stores, but reduces Vitamin D production, so body increases cholesterol production to compensate. But when move to a cold climate get a double hit - don't get enough Vit D from reduced sunshine, and have too much cholesterol. Vit D prevents rickets and also seems to have a role in reducing cancers.

Different environments impose different evolutionary pressures, producing distinct genetic traits that in certain populations give varying resistance to disease

Parasites manipulate their hosts:

The Guinea worm burns flesh as it seeks exit from its host, forcing it to cool running water to ease pain. The worm then lays eggs in water, which is drunk by the next victim. (Jimmy Carter-led campaign has almost eradicated this worm from West Africa)

Rabies virus infects brain and makes host more aggressive, and triggers swallowing reflex, so mouth foams with saliva which is full of virus ready to pass on with next bite.

Liver fluke produces eggs which are eaten by snail and excreted in slime which is eaten by ants. The ants brain is colonised, and they are forced to climb to the highest point around, which is usually a blade of grass, where they get eaten by a sheep/herbivore for the next stage of life cycle.

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toxoplasma gondi virus only sexually reproduces in cats. But when it gets into mice, it blocks their fear of cats. (What's term for a fat, lethargic mouse which is attracted to the smell of cats? .... Dinner)

But the plot thickens, because scientists now think humans are infected by the same virus (our brain chem is very similar to rats and mice). It makes women become more outgoing and warm-hearted, .... and promiscuous. It makes men more aggressive and less rule-abiding.

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Every infectious agent has same goal - to survive and reproduce to infect new hosts.

So if they achieve that by close contact of hosts, it needs them to be healthy enough to move around - so the cold virus doesn't prevent mobility. But if it does it by hitching a ride on an intermediate host (such as malaria virus on mosquito) there is no such pressure for it to evolve a benign form; in fact if the host is incapacitated and awash with parasites there is a better chance that mosquitoes will find victim.

Same with cholera - for most of history there has been evo advantage in virulence, because more diarrhea meant more copies of bacteria on way to new hosts. If sewage flows freely into rivers that people use for washing or drink from, it can multiply freely, with no concerns about using up hosts. But if the water supply is protected, can't transmit that way, and so only the less virulent forms, which leave victim able to stagger around and touch other people, should be able to reproduce. When scientists have analysed outbreaks of cholera in various S American countries, this is what they found.

When an organism is under pressure, it evolves very fast. Expts with corn with too much heat or too little water - the challenge forces the organism to throw the evo dice, come up with a whole lot of alternatives. Idea that many variations sitting passive in genes, with potential to be used in emergencies. For example, expts on e. coli, which normally cannot digest lactose. So they deprived it of everything but lactose. Most died, but massive incr in mutation rate, and eventually strain emerged that could stay alive on lactose, then each generation improved on that capability.

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