This is the right tip:
1) pick any symtoms
2) It Doesn't matter where u start except in these countries:Argentina,Peru....
3) Buy the three resistance:Heat I,Cold I,Moisture I
4) Delete ur symtom
5) wait until every country is infected
6)Buy all the symptons in Tier 4
Wait until everyone get sick and wait until they die......
Finished it last year easy
HINT:put a female bug on sales then take it out so it can breed again......and if u have a sick bug put it on the sales 2 and it will be healthier