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Sickle cell DISEASE by contrast refers to an individual with two sickle genes and this is a serious condition since there is extensive sickling in a person who is otherwise normal and lacks, for example, malaria. The survival rate of such individuals in Jamaica (where 10% of the population have a sickle gene and 1% two copies) is 57 years, a good 10+ years below normal—nor is life pleasant or easy. This is the only crudity in the same, the failure to ameliorate the negative effects of two copies.

It is correct as Peter says, that relatively recent human invasion by malaria (~10,000 years ago) was probably in response to large population increases due to agriculture and so on. This short time period has also prevented much selection to ameliorate the sickle cell disease. It is not true, however, that “it and many other crude adaptations” result from the “ability of the malarial parasites to outwit the immune system”. First of all, other “adaptations” are no more crude than the sickle trait. For example, the Duffy antigen, a surface protein on red blood cells, is designed to spot Plasmodium and prevent entry. And the parasite does not “outwit the immune system”. It is one of the very few Protozoa (or for that matter bacteria) that live WITHIN cells and thus escape the immune system entirely (except see below). Viruses live inside cells, precisely why we can’t use antibiotics against them.

And here is a joke for you. Bacteria, as we know, are rapidly evolving resistance to almost all antibiotics but on many tips of the branches of the bacterial bush, species are evolving that DEPEND on antibiotics for their food—stop treating them with antibiotics and they die of hunger!

Plasmodium reproduces within its red blood cell and the offspring are released into the blood as the cell bursts. Now the immune system responds strongly and all hell breaks loose, fever and chills, bed-ridden and sometimes dead. When given a choice, mosquitos preferentially bite sick people, perhaps because they are too weak to swat the mosquitos away. Once in a mosquito, they do nothing to harm it other than congregate in its mouth near the biting organ, the better to arrive new human.

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