[relaxed state]

You could say you were typing on your Laptop doing homrwork or something then edit some blood onto the keyboard as if you had a severe nose bleed. What you would need to tell them is you have some type of disease that causes your blood to be a hassle to work with. It sounds shitty how I put it into words but remember that how you frame it with your story will determine it all.

So key takeaways:
1. Nobody wants to work with contaminated equipment, EVER,
2. Frame it
3. Get well rounded in a photo editing program & take pictures of your SE jobs just in case you want to do them again. You'll have pictures that aren't available online.

None of this is "hard" to do per se you just have to want it more than the Company wants to deal with you.