IIRC the winning strat for the kill the world in under 100 days consistently was buy a lot of damaging highly infective traits but before it has time to be fatal sell some damaging traits and then once you have a really solid base just swap over to harmless symptoms. Go full lethal only after infecting everyone. Even madagascar only holds out for like 30-60 days and no one closes anything down if you go fast but non lethal. So basically making use of selling traits is the key to cheesing the system and killing everyone. Thank god covid 19 can't just spend points and become ultra lethal all of a sudden.
I gave up on that one. I think I was at day 104. ---