act of god acts of god ailment autoimmune bacteria cdc center for disease control contagious contagious cough coughing coughs disease diseases epidemic epidemics epidemy fever feverish fevers flu force majeure illness illnesses infect infected infection infections infectious influenza maladies malady pandemic pandemics pandemy plague public health pulminary quarantine quarantines respiratory sickness vaccination vaccinations vaccine vaccines viruses world health organization calamity calamities