Fish like people can also catch viruses. In the fish the virus shows up as an irregular growth on the skin or fins of the fish. The growth (or wart) is usually grey, white or flesh coloured.
Occasionally the growth becomes extensive and takes on the colour of the surrounding skin. It looks something like a rough raspberry at this stage.
Fish pox is most common in koi and other coldwater fish and to a lesser extent will affect tropical fish too. It can develop and subside only to re-appear later. A bit like a cold sore on your lips. It is not very infectious and rarely spreads to other fish.
Treatment of fish pox
As in people there is no real cure for fish pox because it is viral in nature. Raising the temperature up by 15F will usually cause the symptoms to temporarily subside.
It is not dangerous to the fish but it is unattractive in appearance.