I'm forever catching up on reading old Future Surveys and the June 2002 has a run of books on health issues, including Secret Agents: The Menace of Emerging Infections by Madeline Drexler (Amazon US and UK). The summary of this includes this paragraph:
If a 1918-force epidemic hit us today, CDC officials estimate up to 200 million Americans would sicken, 45 million would need outpatient medical care, 800,000 could be hospitalized, and up to 300,00 could die (contemporary air travel would surely hasten the global spread of a new virus, from four months in 1918 to perhaps four weeks today, and urbanization would further speed transimission once the virus arrived in a new locale).
Quite a wild card. As a comparison, this World Health Organization press release states that 200 million are affected by flu in the northern hemisphere each year, and 20,000 Americans are killed. Epidemiology is one of the many, many things I'd love to learn more about as it sounds fascinating.