It's mentioned on the main page that the Parasite shouldn't kill its host, but just feed off of them, yet you're still trying to kill people if you play as a Parasite. However, a lot of lethal diseases are lethal to humans because they initially evolved to infect and thrive inside animals. Name any disease that wiped out massive amounts of humans at some point in history and odds are it's something that evolved to jump the species barrier. Perhaps the same is true for the parasite we play as—what it does to humans to kill them wouldn't kill its intended host.
Another often-mentioned bit of Fridge Logic is that every infected person will suffer the same symptoms the instant you evolve them, instead of the new, lethal strain having to out-compete the old one. However, there is the concept of latent symptoms, which are symptoms that appear long after infection. Perhaps the plague already has the lethal symptoms in its genetic code from day one, and by evolving them, what we're actually doing is telling the plague to start expressing those lethal genes.
Z-COM's status as an Increasingly Lethal Enemy makes sense. Z-COM is an X-COM Expy, and the X-COM series is well-known for its typical Early Game Hell; you start with no special equipment and rookies who possess no abilities, and end up with Colonels with varied skills and equipment that fits your preferred play style. Since the Necroa Virus essentially plays as a Perspective Flip, Z-COM is a small threat at first, but becomes more and more lethal.
While not the most deadly disease in the world, the common cold is a widespread infectious disease. Infecting more people than the common cold is quite a feat worthy of it being used as Hiroshima as a Unit of Measure of Infectivity.
When playing the Neurax Worm, occasionally, the news ticker will announce bans on long hair, hats, and other such headwear. It feels like a total non-sequitur to show that your influence has even hit politicans, but think about it; where is the Neurax Worm? What would get in the way of where the Neurax Worm is supposed to go? And what would be adding an unneccessary (to it) amount of pressure and/or warmth to the place the Neurax Worm resides in? Suddenly, the news messages come across a lot more like the Neurax Worm abusing the legal system to just create a more hospitable environment for itself, which is as terrifying as it is utterly hilarious.
In the Necroa Virus plague, the description of the symptom Anaerobic Resuscitation basically says that brain cells now do anaerobic respiration, destroying the infected person's mind. The simple fact that this symptom exists may imply that the previous symptom (Cytopathic Reanimation) brings the infected back from the dead, still with their memories of their previous lives and with some self-awareness ... Imagine how it must be: You died of a mysterious virus and then you came back... now under the control of that same virus, being forced to kill and eat everyone around you, and you can't do anything about it!
Alternately, as Anaerobic Resuscitation is the only zombie symptom that increases Lethality (of the virus itself), it could be effectively frying your brain with lactic acid while you're still alive, leaving you an Empty Shell before you even turn into a zombie, and priming the altered braincells to develop the newly created zombie into something worse (i.e. unlocking the rest of the zombie symptoms). Either way, it's one of the most horrifying symptoms of the Necroa Virus if you think about it too hard.
Or make belly landings. Who says the Worm can't control people to be skilled pilots?
You don't need a runway to land a plane, just a sufficiently long, flat stretch of land.
The weirdest part is the fact that nobody seems to care about a plane landing even after the borders are closed.
Air smuggling is a thing, and successful smuggler pilots are presumably skilled in evading the authorities, else they wouldn't be successful smuggler pilots. The smuggler is ultimately an expendable asset for the worm, and in the end his fate matters little as long as he starts spreading the worm. Even if he gets caught/killed, by that point the damage has already been done.