After trying for about a week I managed to beat this game on realistic and then came up with a walkthrough. It's not full-proof and still requires some luck, but using this it's about 6 times easier to win.

1) Pick Virus.
2) Start in Canada, New Zealand, Cuba, Greenland, or Madagascar.If you do not get there restart.
3) Sell starting symptoms, if any, and buy sneezing (unless it is what you started with, just keep it).
4) Buy 1-1-1-0 resistances but no transmissions (they trigger closures).
5) Once three countries are infected, sell sneezing to get rid of visibility.
6) Wait until all countries are infected. If Madagascar closes its shipyards or is not infected in 30 days, restart. 7) Buy all four drug resistances, sneezing, coughing, and vomiting.
8) After a few days, unlock tiers 2-3, keeping whatever symptoms it gives you.
9) Buy fever, fatigue, diarrhea, pulmonary edema, and hypersensitivity.
10) Wait until everyone dies.