coronavirus variant outbreak

coronavirus variant omicron subvariant covid19 coronaviruses influenza background sars flu strain outbreak pandemic xbb fever diagnosis symptoms sarscov2 coronavirinae microbe virus infect epidemic lethal pathology pathogen virology sickness bacterium health infectious retrovirus superbug immunity human pathogenic medical science biology antibiotics bird avian vaccine microbiology render concept bacterial infection illness contageous treatment covid-19 dangerous cases risk disease cells