There are two main types of Viral Infection you're likely to see in fish. 

Carp Pox – Carp pox is a herpes-like virus which affects carp and goldfish. It resembles white/grey blobs or warts which appear waxy. The pox growths usually develop around the fins and tail of the fish, but can also grow around the head. Carp pox tends to appear in spring and, less often, in autumn. It is more likely to affect a fish with low immunity, so avoiding stress in the pond can help.The virus is completely untreatable, some fish keepers advise to scrape the growths away, but this can badly damage the epidermis and should not be practiced.Although the growths can be very ugly, the virus does not usually affect or harm the fish. As The fish’s immunity levels improve and the water temperatures rise towards summer, the condition usually clears up. Carp pox is quite rare in adult fish.

Lymphocystis – Lymphocystis is a viral infection very similar, but much rarer than carp pox. This virus can affect any pond fish, and is not influenced by water temperatures or fish health.Like carp pox, this virus doesn’t usually harm well cared for fish with low stress levels, unless quite a large growth develops on an area such as the mouth or gills. It just looks unpleasant!The lymphocystis infection starts as a small cluster of spots. As it develops, the growth resembles a small cauliflower or raspberry, it is normally whitish during the early stages, but begins to turn red or purple. At this stage it is advised that you should treat the fish with a general tonic to reduce the chances of the fish developing a secondary fungal or bacterial infection.

The viral growth usually sheds away after around a month, and the fish should make a full recovery.Lymphocystis can be contagious. It may be beneficial to other pond fish if the infected fish are removed to a quarantine tank until the virus passes, and a partial water change can be carried out on the pond to dilute any waterborne viral particles.