For other walkthroughs, click PLAGUE INC MEGA BRUTAL WALKTHROUGHS tag at bottom of this post.
Android only, on my tablet - I don't know if it works on Xbox One or any other device.
I found Nano Virus so hard, I must have tried it about 30-40 times, tweaking the sequence a bit each time... eventually I did it this way, and it worked.
Here's what I did:
DNA Genes - Cytochrome Surge
Travel Genes - Aerocyte
Evolution Genes - Patho-Statis
Mutation Genes - Genetic Mimic
Environment Genes - Extremophile
Start in India (I've tried Egypt & China - not too bad - Brazil and Russia - hopeless)
As you know, Nano Virus is difficult because you have to worry about this wretched kill switch thing, while getting it to infect enough to keep the DNA points coming. Making subtle changes to the order in which I activated all abilities, transmissions, etc, this is what worked:
First, activate in this order:
Code Fragment Interception
Code Segment Interception
Radical elements stablised
Replication Factory Overload
I got Air 1 first, as I got the message about new air filters on planes
Cold resistance (CR) 1 (because China was infected, which is near Russia)
Drug Resistance (DR) 1 (as it was getting around countries with good medical research facilities)
By now the cure was up to 77% so I got Encryption Breached to take it down.
I devolved Nausea, which had mutated itself, giving me another point
The cure was back up to 72% so I got Broadcast Interceptor Overload.
Genetic Hardening 1 and 2
I don't usually get lots of symptoms but it wasn't infecting very quickly. The first two to help the cold countries, the last two lead to Paralysis.
Genetic Reshuffle (GRes) 1
Paralysis (Good for knocking the cure back)
Still not infecting very fast so...
Paranoia (also knocks cure back nicely)
Next, it automatically mutated Sneezing - handy!
I got Bird 2
Next, my nano-virus automatically mutated Coma (and at last Greenland got infected!)
Then it automatically mutated Total Organ Failure - that worried me a bit as it was too early. Getting TOF can be curtains, because it kills everyone off before it's infected them all; I had to act quickly and get the rest of them struck down with my Nano-Virus as fast as poss.
I usually play on Fast Forward but went down to normal speed at this point because I thought I was actually going to do it, and didn't want to make any wrong decisions
Livestock 1 (logical because West Africa and Greenland were still looking healthy, and there are probably a whole bunch of camels and buffalo and elks and things wandering about in those two countries)
Mutated Seizures on its own.
I got Insanity - that knocks it back too - I like to think of all those researchers going nuts.
Bit of an anti-climax to do Bio-Weapon afterwards - I got it straight away, too easy. Just remember to modify with Patho-Stasis, and get all the six things at the top right hand side of the Abilities screen early on.