The Neurax Worm music is a different story. A piano plays to an incredibly creepy bass, and for added measure, random noises that sound like worms play for Paranoia Fuel.
The Necroa Virus music. Sounds definitely like something you'd hear in a George A. Romero film.
Some sound clips can play above the music, such as children laughing and singing, medical equipment, coughing...real subtle.
In the Steam version of the game, occasionally you can clearly hear a child singing parts of "Ring Around the Rosie" near the end of the game. (More specifically, "...ashes, ashes, we all fall down...")
In the Necroa Virus soundtrack a respirator and cardiograph provide the backbeat for the music..
The Neurax Worm is a Puppeteer Parasite taken Up to Eleven. At first it'll start off harmless, as described by scientists. But by the time you've upgraded, humanity won't be able to react quickly enough to stop the infection spreading, and the cure can be stopped in a single upgrade of a symptom. The image of the plague also changes from a Petri dish to a worm sitting on a brain with tendrils extending inside it, eventually expanding to all the brain, turning it a sickly orange, and causing flesh bubbles to pop out. The worm seems almost intelligent in its symptoms, such as causing people to fly to a specific country in order to infect it, even if that country has closed off its airports (or has no airports). Another symptom involves violent attacks against anyone who isn't infected, and active attempts to stop cure research. The starting blurb mentions that the worm has been dormant for millennia only to be uncovered by excavation. Additional news mention people worshiping the worm, achieved with the Transcendance symptom. Notice how it gives a massive boost to Severity.
The Necroa Virus is your typical zombie affair, except Our Zombies Are Different. This plague can be the result of human experimentation gone wrong, a plague from ancient Egyptian tombs and mummies, or from the remains of Chernobyl, infecting animals. The symptoms can become remeniscient of Left 4 Dead style zombies, and the on-screen news is horrifying. To list some examples:
Smallpox and the Black Death are scary on their own. What's worse is they begin with high lethality, so if you don't spread quickly, you lose. The symptoms are also highly disturbing, and are the real deal.
Frozen Virus is a more apocalyptically frightening scenario. When you begin, you have to evolve DNA Repair in Abilities. When it's fully evolved, you'll get◊ the ability to "reshape human temperament". The meaning of that soon becomes clear as you start killing; one thing you'll notice is that some of the symptoms are borrowed from the Necroa Virus, leading up to this.◊ For those to nervous to tackle the link, it's Neanderthal Regression; ie. mutating everyone back into primates. The win condition for the Frozen Virus is borrowed from the Neurax Worm: Infect everybody, evolve the 2 final lethal symptoms, and watch as this begins◊ and ends◊. It's nota pretty thing to see.
The humble Parasite's symbiosis ability, which makes itself less severe by merging with host DNA and finally burrowing into the host's brain to escape detection, is completely horrifying. Turn Severity down to zero, get a couple of harmless but infective symptoms like coughing and sneezing, and quietly rack up the environmental and transmission buffs until the world is 100% infected without the disease even being noticed. Wait for enough DNA bubbles to get the DNA for fatal symptoms and hit the kill switch. Humanity won't be able to do a damn thing about it on any but the highest difficulty level.