Why do flu viruses have rna instead of dna? and why do scientists have to make new formulas for flu shots?

RNA is genomes viruses. It caused thousands of death each year. For example Spanish Flu 1918 is the most deadly viruses. Because of deadly viruses that is why scientist have to make new fomulas to control over the flu. Every year, we should have a flu shots. However, there are several viruses have no medication for it, beside anitibiotics, for example, influenza A, B, C or H1 N1 or H3 N2. DNA is genomes of testing. For example, murder case, FBI, Lab, will collect all evidences to find out the killer. |CIS show} they are using DNA.

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