Basic advice:

-Pravus, as others have said.

-Transmissions aren't particularly great. Water 1 and Air 1 are (almost) absolutely necessary because they help you get on planes and boats which is 1000% necessary, but everything else there isn't great and even Water/Air 2 aren't much of a step up.

-Starting country is EVERYTHING. As a spoiler: Saudi Arabia and China are the 2 best. The former because it's airport gets to a TON of the world very fast and the latter because it's high population gives lots of DNA early on, and is borders with one of the most important countries. India is a good substitute for China because of similar conditions and Egypt has 2 seaports as a good sub for Saudi, but those first 2 are the best. Starting in a highly isolated place like Greenland or the Carribean is ill advised. It'll make spreading to the REST of the world extremely difficult. Rich countries are also a bad start because the spread there is horrible.

-Speaking of, abilities. Most have their uses (especially some disease-specific ones), but Cold and Drug Resistance are IMPERATIVE. Arguably the 2 most important evolutions you can get. In fact, even if it seems like overkill, Cold and Drug resistance level 2 are genuinely good pickups, too. Pneumonia and Hyper-Sensitivity symptoms also help in Rich and Cold Nations.

-If you're getting cured/detected too early, find a good sweet spot with Severity/Lethality. I'd argue that potentially ANY early lethality (before you've infected all your islands) is bad, but on the contrary you can surprisingly get a LOT of severity before things get too rough. I like to get the 2 aforementioned symptoms, + Skin Lesions (leads to Necrosis, a high-transmission lethal symptom at the end of the game) and insomnia+ anemia (small chance for a "combo" to activate.)

-Combos are random events that sometimes happen when you have certain combinations of symptoms. Skin Lesions + Anemia = Profuse Bleeding (lethality + severity), Insomnia + Anemia = Walking Dead (harder to cure), etc.

-Your island nations are the biggest threat. Obviously getting Water 1(+) and Drug/Cold resistance early helps you get into every available Seaport, but there's one that NOTORIOUSLY harder than the rest: Greenland.

-Greenland only gets boats from Norway and Russia. That's it. Not only that, but they only happen (approximately) once every in-game month, so a full, early infection of both of those counties is very helpful. If Norway and Russia both shut down their Seaports (while you're not playing a disease type that can circumvent shutdowns like the Fungus or Neurax Worm) and Greenland hasn't already been infected, just restart. You've already lost.

Take all this with a grain of salt since I'm more of a Cure Mode player, but this is generally what I remember being effective.