What is plaguing my pepper plant?

Question

For the last few months, my pepper plant has been continuously suffering from some kind of disease or parasite. I've been hoping it'd recover on its own, but instead, its been progressively getting worse.

As such, I'm coming here with hopes of finding out what exactly is plaguing it. Pictures are attached below, but here some more info:

• location: southern Germany
• plants are indoors directly next to the south-facing window
• my one year older hot pepper plant is showing the same symptoms, but to a FAR lesser extent. I'm not sure if its species is more resilient, if its age makes it more resilient (the suffering plant was purchased this spring), or what else the reason why it's not suffering as much may be. Or perhaps it's not more resilient at all, and just experiencing the problem delayed.
  The mint plant on the same window sill is exhibiting no symptoms at all and is doing just fine.
• I've had problems with fungus gnats in the past, but ever since I've added a layer of sand to my plant pots a few weeks back, that problem has decreased significantly. My pepper's problem, however, has increased in severity since (just as before adding the sand), so that can't be it.
• Watering and fertilising should not be the problem. I use a moisture meter to gauge when to water, and I've been fertilising my plants on a roughly bi-weekly schedule, which the other plants are just fine with.

Images:

Full-size images (text-links in order to prevent inflating the post too much):

• Image 1
• Image 2
• Image 3
• Image 4
• Image 5

Answer 1

Assuming not too much sun in combination with too much fertilizer, I suggest a likely reason could be Tobacco Mosaic Virus (TMV) or perhaps Cucumber Mosaic Virus (CMV) This is especially possible if the illness originates from a newer plant. The hypothesis is based on the light areas (mosaic pattern) on the leaf in image 5.

CMV infected plants also tend to be stunted with those plants infected early in development possibly expressing severe stunting, whereas plants infected at later stages of development may have little, if any, stunting.

Quotation from the CMV link. This can explain the different development/resilience of the disease.

Remove infected plants and any dead leaf, dust etc. You can transmit the virus by touching a healthy plant after touching infected leaf.