Tainted Veins

Who'd have thought putting an experimental clean energy source in your chest might have harmful side effects?

He has veins on his forehead now, which is the official Star Trek code for "This poor bastard is in for a horribly painful transformation and/or death." Seriously, if you ever find yourself in the Star Trek universe and you see this happening to yourself, turn your phaser on yourself! There is no hope if you have a veiny forehead unless you are an alien or a main cast member. Just spare yourself!"

—Sci Fi Debris reviewing Star Trek: Voyager's "Threshold"

In general, the tendency of people suffering some sort of abnormal condition to develop highly visible, off-color veins all over their body or near the distressed areas.

Might be due to catching a bite from someone/thing infected with The Virus, or as a manifestation of a villain's Red Right Hand after willingly succumbing to The Dark Side or The Corruption. It's also possible if the character is suffering a Superpower Meltdown, Power Degeneration, or as a side effect to taking too much Psycho Serum. A vampire, werewolf or other supernatural critter hit with Depleted Phlebotinum Shells inducing a burning reaction (or even just partial sun exposure/blood starvation) may develop Tainted Veins because their glamour is failing. Special mention goes to zombie bites for always creating nasty necrotic veins wherever they bit.

Expect characters with a Healing Factor (or with friends possessing Healing Hands and or a Magic Antidote) to quickly and neatly reverse the appearance of these veins.

See also Volcanic Veins for when they glow. Compare Vein-O-Vision, which is where a character has a power that makes them see the world this way.

This is a Justified Trope (Tropes Are Not Bad): There are toxins that cause venous thrombosis (clotting of blood in veins) which can make surface veins visible and dark - especially if the clotting is in deep tissue in an affected limb.

Examples of Tainted Veins include:

Anime and Manga

Babidi's human henchmen in Dragonball Z have unusually bulging veins without discoloration.
This is one of the warning signs of Tetsuo's imminent mutation near the end of Akira.
Greed in Fullmetal Alchemist gets these whenever he and Ling have a disagreement.
- Ed also gets these temporarily when Pride tries to take him over.
Dark Marik of Yu-Gi-Oh!.

Film

The vampires in 30 Days of Night.
'Dreamcatcher had this, but it was an alien plant growing into the infectee's face.
The vampires in Interview with the Vampire had visible veins on their faces, too, but that was as much due to their creepily transparent porcelain white skin as it was to their veins being full of freshly drunk blood.
Tony Stark's Palladium poisoning in Iron Man 2 displayed these symptoms.
alien-influenced humans in Skyline.
Poison Ivy's toxic kisses cause these to appear on her victims in Batman and Robin.
This happens to whoever Rogue touches in the X-Men movies.
- Mutated Senator Kelly also has them before he dies.
Demoni.
A sign that you are falling under the Big Bad's sway in 5ive Girls.
The poison Necrosis in the live-action version of Black Butler has this effect for a brief time before the mummification sets in.

Literature

From the second-to-latest book in the Dresden Files series, Changes, we are treated to a minor case of this courtesy of Ebenezar.
In the Skulduggery Pleasant series, people possessed by Remnants (evil shadows/sprits) get black veins and lips, but only if the Remnant wants to show itself. Otherwise they just look like normal people.

Live-Action TV

Dark Willow on Buffy.
An episode of Star Trek: The Next Generation had Geordi get infected by a disease that covered him with invisible blue glowing veins. It Makes Sense in Context.
- Borg assimilation does this, with added blueish-grey skin.
- SF Debris notes that if your veins start going weird (and especially on your forehead), you're screwed unless you're a main character.
The 2nd Doctor was up against a "plague" that gave a moonbase crew tainted veins. Eight years later, the same tainted veins gave the 4th Doctor a clue who he was up against.
- In the series 5 two-parter The Hungry Earth/In Cold Blood, Alaya poisons Tony. His veins turn green.
In Smallville, Clark gets 'em when Kryptonite is around... and Kryptonite Is Everywhere.
Happens a few times in Supernatural, more notably when Castiel is possessed by the leviathans.

Tabletop Games

The Seventh Soul in GURPS Villains has black veins all over its body. These are its only persistent identifying feature, since it reincarnates.

Video Games

After Beatrice dies in Dantes Inferno, her spirit form has dark veins... all over her naked pale chest.
In Deus Ex and other games set in the same universe, nanotech augmentation gives you this. Although you're far from sick... Actually, you are sick and your augs and Gray Death are one in the same, you are just immune to the bad symptoms.
In Dragon Age II, this is one symptom of the Darkspawn corruption that finally kills Wesley.
- And Bethany, if she dies in the Deep Roads. Or anyone who dies via the Taint, of course.
In Killzone, Helghast have these, as a result of the first settlers trying to adjust to Helghan's sickly atmosphere and hundreds of years worth of forced evolution.
In Knights of the Old Republic, ash-pale skin with blue veins signifies your character going completely darkside.
Happens to Leon in Resident Evil 4, during a dream sequence after he's infected with the Las Plagas parasite. Fortunately for him, he's able to find a cure before anything like that starts to happen for real.
In Metroid Prime 3: Corruption, each time Samus absorbs a Phazon Core, her veins become more noticable; bigger, longer, bluer.
Spore has this as a variant of coloration for your creature.
Taken Up to Eleven in Prototype and Prototype2. Alex Mercer's and James Heller's bodies are covered in thick, pulsating red and black veins when they unleash their Blacklight-virus powers.

Web Comics

In Wake the Sleepers, Locke's hand.

Web Original

In the Global Guardians PBEM Universe, when the supervillain Virus injects his victims with the mind-control serum, their veins turn dark green around the injection site.
- Cyborg supervillain La Constructeuse has this sort of thing going on with her own body, but the vein-tracery looks like circuitry.

Real Life

Tainted Veins (or the appearance thereof) are a common consequence of venous thrombosis - a blood clot lodging in a vein and obstructing flow proximal or "upstream" (i.e. back to the heart) from that point on. With the main vein blocked off, smaller collateral veins frequently become engorged with backed-up blood trying to find an alternate path around the blockage. As a result, all the veins distal or "downstream" of the clot will swell (sometimes to grotesque proportions), and those that were close to the skin to begin with will become easily visible on the skin surface as dark purple or blue lines.
- A slightly nastier version of the same phenomenon occurs in the context of central retinal vein thrombosis - a blood clot blocking off the main vein draining blood out of the eye. The overall appearance is of an entire eyeball (retina, globe and all) with a case of bloodshot turned Up to Eleven. Cue Eye Scream and/or Squick as appropriate.
- In a crossover between Tainted Veins, Blood From the Mouth and High-Pressure Blood, consider portal hypertension. In chronic alcoholism and a few other conditions of massive liver damage, the liver becomes so scarred that blood vessels can't get through it anymore, and as a result the entire blood supply starts backing up, causing high blood pressure in the very fragile veins of the upper GI tract (esophagus and upper stomach mostly). The visible manifestation of this situation, called "caput medusae" ("Medusa's Head"), presents as a dark, snakelike pattern of veins across a swollen abdominal surface. The deeper cousins of these swollen veins, called "esophageal varices," can easily rip open, spilling the patient's entire blood volume into his/her GI tract in a matter of minutes. If you're the physician or nurse dealing with this situation, You Can Panic Now.
While not strictly vascular, gangrene can cause reddish or greenish streaks running away from the site.