#completely fine

No, seriously. They're allergic to nanobots.

Speedsters have absolutely insane metabolisms, which means that they have an absolutely insane immune system. They don't get sick. Ever. Their immune system works at warp speed and takes out germs the second they enter their body. Call germs 'the Rogues' because they're getting tackled by super speedy blurs before they can even think about causing issues.

Okay, so they have a great immune system and don't get sick ever. What does this have to do with nanobots?

Great question! When nanobots are injected into a speedster's body their immune system sees them as a threat. Only problem? It doesn't matter how fast or efficient their immune system is, their body can't destroy a bunch of tiny metal robots.

Because their bodies can't fight off the nanobots they start to display typical cold/flu symptoms instead. Vomiting, fever, runny nose, coughing, being tired, ect. The nanobots aren't causing this reaction. Their own immune system causes this reaction. The fever is the bodies attempt to kill off the 'germs'. The vomiting, runny nose and coughing is the body's attempt to expel the 'germs'. They feel tired because their body is putting everything into fighting off the 'infection'.

In a normal person the nanobots wouldn't even be an issue because they'd be able to avoid detection. They can't avoid detection in a speedster body because their immune systems are dialled up to 500 out of 10.

As a result you get instances like this:

(Inertia had injected Bart with nanobots and Bart had a reaction)

Just an FYI for people because this is extremely fun and versatile information. Especially because none of the speedsters are really aware of this and it doesn't kick in right away. I could totally see a situation where a mission requires nanobot injections and mid mission the speedster goes down out of nowhere. It's also great if you want to do a stereotypical sick fic or something and want to get around that pesky speedster immunity.