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" The four known for targeting humans are transmitted from person to person by Anopheles mosquitoes. These four parasites possess wondrously complicated life histories, encompassing multiple metamorphoses and different forms in series: an asexual stage known as the sporozoite, which enters the human skin during a mosquito bite and migrates to the human liver; another asexual stage known as the merozoite, which emerges from the liver and reproduces in red blood cells; a stage known as the trophozoite, feeding and growing inside the blood cells, each of which fattens as a schizont and then bursts, releasing more merozoites to further multiply in the blood, and causing a spike of fever; a sexual stage known as the gametocyte, differentiated into male and female versions, which emerge from a later round of infected red blood cells, enter the bloodstream en masse, and are taken up within a blood meal by the next mosquito; a fertilized sexual stage known as the ookinete, which lodges in the gut lining of the mosquito, each ookinete ripening into a sort of egg sac filled with sporozoites; and then come the sporozoites again, bursting out of the egg sac and migrating to the mosquito’s salivary glands, where they lurk, ready to surge down the mosquito’s proboscis into another host. "
― David Quammen , Spillover: Animal Infections and the Next Human Pandemic
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" I PALE HORSE 1 THE VIRUS NOW known as Hendra wasn’t the first of the scary new bugs. It wasn’t the worst. Compared to some others, it seems relatively minor. Its mortal impact, in numerical terms, was small at the start and has remained small; its geographical scope was narrowly local and later episodes haven’t carried it much more widely. It made its debut near Brisbane, Australia, in 1994. Initially there were two cases, only one of them fatal. No, wait, correction: There were two human cases, one human fatality. Other victims suffered and died too, more than a dozen—equine victims—and their story is part of this story. The subject of animal disease and the subject of human disease are, as we’ll see, strands of one braided cord. The original emergence of Hendra virus didn’t seem very dire or newsworthy unless you happened to live in eastern Australia. It couldn’t match an earthquake, a war, a schoolboy gun massacre, a tsunami. But it was peculiar. It was spooky. Slightly better known now, at least among disease scientists and Australians, and therefore slightly less spooky, Hendra virus still seems peculiar. It’s a paradoxical thing: marginal, sporadic, but in some larger sense representative. For exactly that reason, it marks a good point from which to begin toward understanding the emergence of certain virulent new realities on this planet—realities that include the death of more than 30 million people since 1981. Those realities involve a phenomenon called zoonosis. A zoonosis is an animal infection transmissible to humans. There are more such diseases than you might expect. AIDS is one. Influenza is a whole category of others. Pondering them as a group tends to reaffirm the old Darwinian truth (the darkest of his truths, well known and persistently forgotten) that humanity is a kind of animal, inextricably connected with other animals: in origin and in descent, in sickness and in health. Pondering them individually—for starters, this relatively obscure case from Australia—provides a salubrious reminder that everything, including pestilence, comes from somewhere. "
― David Quammen , Spillover: Animal Infections and the Next Human Pandemic
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" As the new century began, AIDS researchers pondered this roster of different viral lineages: seven groups of HIV-2 and three groups of HIV-1. The seven groups of HIV-2, distinct as they were from one another, all resembled SIVsm, the virus endemic in sooty mangabeys. (So did the later addition, group H.) The three kinds of HIV-1 all resembled SIVcpz, from chimps. (The eventual fourth kind, group P, is most closely related to SIV from gorillas.) Now here’s the part that, as it percolates into your brain, should cause a shudder: Scientists think that each of those twelve groups (eight of HIV-2, four of HIV-1) reflects an independent instance of cross-species transmission. Twelve spillovers. In other words, HIV hasn’t happened to humanity just once. It has happened at least a dozen times—a dozen that we know of, and probably many more times in earlier history. Therefore it wasn’t a highly improbable event. It wasn’t a singular piece of vastly unlikely bad luck, striking humankind with devastating results—like a comet come knuckleballing across the infinitude of space to smack planet Earth and extinguish the dinosaurs. No. The arrival of HIV in human bloodstreams was, on the contrary, part of a small trend. Due to the nature of our interactions with African primates, it seems to occur pretty often. "
― David Quammen , Spillover: Animal Infections and the Next Human Pandemic
