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It is a pestilence that has harried civilizationssince at least the time of Homer.What's more, it has done so with such routineperiodicity that, in our modern age of annual05inoculations, the enduring danger of thisdisease has grown all too easy to take forgranted. Influenza owes its name to physiciansof the Italian renaissance, who believedit was caused by inauspicious astrological10"influences." Today, of course, we know it tobe the result of infection by one of severalclosely related strains of virus. However,unlike other viruses for which vaccinesare available—several of which, through15tenacious public health efforts, have beeneradicated worldwide—influenza remainsa perennial menace, and due to the uniquenature of its genome, is unlikely to ever becompletely conquered.20Traditionally, outbreaks of influenza areclassified as either "epidemic," in which theincidence of the disease increases significantlywithin a given community, or "pandemic,"in which the incidence increases25over a much larger region, such as a continent.While superficially the distinctionmay seem arbitrary, in fact it reflects twowell-delineated facets of the influenza virusreplication process. In the Northern hemisphere,30"flu season" spans from November toApril, and represents an annual recurrenceof influenza epidemics among communitiessituated in this part of the world. Pandemicoutbreaks, though not nearly as common,35also seem to follow an approximate epidemiologicalpattern, typically occurring aboutthree times per century. In the 20th century,these outbreaks included Spanish Flu in1918, Asian Flu in 1957, and Hong Kong Flu40in 1968. Of the three, Spanish Flu was byfar the most devastating. With an estimatedmortality as high as 100 million, its deadlinesswas on par with that of the infamousBlack Plague, which ravaged Eurasia in the45Middle Ages."Antigenic drift" and "antigenic shift" arethe two chief processes through which influenzacircumvents our adaptive immunity,and are thought to be the causes of epidemic50and pandemic influenza, respectively. Tounderstand these two processes, it is necessaryto have a working knowledge of thevirus itself. There are three known species ofinfluenza virus—influenza A, B, and C—each55of which consists of eight segments of RNAcontained within a protein capsid, whichin turn is surrounded by a lipid envelope.Collectively, these RNA segments code foreleven proteins; two of which, upon synthesis,60are expressed on the envelope's exterior.These two proteins are known as hemagglutinin(HA), and neuraminidase (NA). Interms of the viral life cycle, HA is responsiblefor attaching to sugar residues that coat the65cells of our respiratory tracts. Once the virushas infected a cell and replicated within itsnucleus, NA cleaves these residues, allowingthe virus to spread further throughout thebody.70Because HA and NA are the outermostviral proteins, it is specifically against thesetwo "antigens" that our white blood cellscreate antibodies. Furthermore, amongthe diverse strains of influenza, genetically75encoded differences exist in the types ofHA or NA expressed. This allows scientiststo sub-classify strains based on the specificantibodies produced against them. Forinstance, the H1N1 strain was responsible80for both Spanish Flu, as well as the Swine Flupandemic of 2009, while H5N1 caused theAvian Flu epidemic of 2004.Random point mutation to the genesencoding HA and NA is one way in which85these subtypes evolve, and can, moreover,interfere with the efficacy of our antibodies.The aggregation of many point mutationsover time is referred to as antigenic drift, andeventually results in renewed vulnerability90to viral strains against which an individualwas previously immune. Notably, influenzaA lacks the ability to proofread and correctits genetic material during replication, andas a result, is prone to a much higher rate of95mutation than other species of influenza.For this reason in particular, influenza A isresponsible for the vast majority of annualepidemics.To date, 16 HA and 9 NA subtypes have100been identified, only a fraction of which arecurrently infectious to humans. However,because the influenza genome is split intosegments, when an animal—a bird, forinstance—is co-infected with a strain specific105to its species, as well as one capable ofinfecting humans, the segments may becomeintermixed during replication in a processcalled "viral reassortment." When the genesimplicated in reassortment include either HA110or NA, antigenic shift occurs, and the resultingviral particles will express novel proteinsto which the entire human race is vulnerable.
The table shows, for each human outbreak of influenza, relevant epidemiological data, and the viral subtype involved.
Human Influenza Outbreaks
1. The structure of the passage is best described as a
2. Lines 12-19 ("However . . . conquered") most strongly suggest that influenza
3. As used in line 21, the word "epidemic" would best describe which of the flu outbreaks in the table?
4. Based on the passage, would antigenic drift or antigenic shift result in greater fundamental changes to genetic structure?
5. Which option gives the best evidence for the answer to the previous question?
6. The primary purpose of the paragraph in lines 83-98 is to
7. As used in line 92, the word "ability" most closely means
8. Given the data in the table, which of these flu outbreaks most likely resulted in the greatest number of deaths?
9. Based on the table and the passage, which flu outbreaks (given by year of occurrence) would most likely result in the human body producing similar chemicals to fight them?
10. Which option gives the best evidence for the answer to the previous question?
11. According to the information in the table, which of these options gives the most logical possible reason that the flus of 2005 and 2013 resulted in relatively few cases?
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