Fictional British PMs are frankly boring from this point of view - you're basically choosing between Alan B'Stard, Jim Hacker, Harriet Jones, Lord Ruthven (Anno Dracula), Harold Saxon (AKA The Master), and Francis Urquhart, with most of the rest nonentities.
On the whole I prefer Harriet Jones for this one - the Master would be more likely to start the epidemic, Lord Ruthven is a vampire so really not too worried about a human disease, B'Stard and Urquhart would be too busy profiteering to do much to help, and Hacker would be concerned but ineffective. Harriet would ask the Doctor for help, and if it isn't a fixed point in time he'd probably sonic up some sort of cure..
Fictional American presidents are more interesting - there are more of them that aren't just nonentities, and there's a slightly wider range of personalities. i've whittled my list down to six again: Merkin Muffley (Doctor Strangelove), Jack Ryan (Tom Clancy novels), Taffy Dale (Mars Attacks), Josiah Bartlet, Lex Luthor, and Robert Kinsey (Stargate series).
Muffley and Kinsey are both tools of the military-industrial complex so probably not good choices for this one. Jack Ryan would unfortunately try to resolve this by declaring war on someone, which unfortunately doesn't really work if there isn't an enemy behind the epidemic. Taffy Dale would probably be good for post-disaster recovery (in Mars Attacks she succeeds her father who is killed by the Martians) but has no track record in crisis management, other than surviving them. Bartlet would try to throw the resources of the government at the problem, but run into problems with Congress etc. and only achieve a small part of what he wants. Which unfortunately leaves Lex Luthor as the strongest contender - he's a genius, he understands science and industry, and he won't hesitate to be ruthless if he needs to be. He'll probably try to blame Superman, of course, but he'll also throw huge resources at the problem because nobody beats Lex Luthor - and because his companies can make vast profits from selling the cure!
Any alternatives I've missed?
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