Yes it's possible for it to kill billions, it's happened before after WW1 and that close proximity problem which allows rapid transmission is FAR worse now than it was even then. Further more the warm, moist air inside shopping centres and airports and train stations allows respitory viruses to hang in the air far longer and therefore infect massive amounts of people, just look at the common cold. On the treatment side we have very little that there wasn't available during the Spanish flu epidemic, there is no silver bullet like antibiotics for a virus. Tammiflu works on some strains but is compleatly useless on others so your back to supportive care and even if Tammiflu DOES work only those at the very start of an epidemic will be administered it, once it gets going it will all be saved for health care workers and essential services and even THEN Australia's stock pile will only last either a week or a month (it's a while since I did pandemic preparedness sorry). It's actually one of the drugs under the direct control of the department of health and which you require permission to distribute