Viruses

Viral infections. A virus isn't living in the usual senses of the word, although that Latin "vir" element in "virus" suggests the spark of life and strength. It's the same "vir" that makes a poison virulent or puts the spirit in a virtuoso. A virus is no more than a fragment of genetic nucleic acid enclosed in a protein shell.
 
A virus becomes an infective agent because it must co-opt the metabolism of a cell in order to get itself reproduced. Phrasing it like that is misleading: there's no more will on the part of the virus than there is some strategy at work in a molecule that's engaged in a chemical reaction. Virus infections in fishes are untreatable.
 
Lymphocysti — and immune systems. We act sometimes as if the only viral disease of tropical fish were Lymphocystis, an iridovirus that causes a single infected skin or fin cell to swell into a white warty nodule that's so large we can't avoid seeing it with the naked eye. Quite often Lymphocystis affects a fin, perhaps with a single white cyst, perhaps in a cluster that's often described as cauliflower- or raspberry-like.
 
Though its effects are obvious, Lymphocystis is neither particularly virulent nor especially contagious. Other fish may single out an individual afflicted with a Lymphocystis nodule, however, and react aggressively.
 
Lymphocystis occurs globally; it affects marine and coldwater fish as well as freshwater tropicals. I would never cut it out of a fin.
 
Link. The best, most succinct capsule internet article on lymphocystis was a pdf file at Aquaculture Asia, the e-magazine of NACA, (Network of Aquaculture Centres in Asia-Pacific). In it, Lymphocystis is called common, chronic, and benign, affecting only skin and fin tissues. The author warns that other fish may pick at the enlarged cell.
 
Other viruses. Many other viruses affect fish. It's possible that the "Angelfish plague" that first decimated commercial stocks of angels in 1986 and went on to attack severums and other cichlids is a virus, rather than any of various bacteria that have been associated with it.
 
Another iridovirus, which acted within 24 to 48 hours, was suspected of causing mass mortalities in Florida pond-raised Trichogaster gouramis: (described in "Iridovirus in Gouramis" at the University of Florida IFAS site, where you might also read the "Introduction to Viral Diseases of Fish").
 
The actions of viruses are normally more subtle. The range of other viruses that are pathogens of fish is an idle question for us non-professionals really. Either the fishes' own immune system will successfully combat a viral invasion — or it won't. That's the purpose of phagocytes released into the bloodstream, whether in fish blood or ours: they identify foreign DNA, engulf the stranger in amoebalike fashion, and digest it or escort it out of the system. However, even unsuccessful viral invasions can stress the immune response and leave the fish vulnerable to subsequent bacterial infections — not that bacterial infections are directly treatable either.
 
Some manufacturers are only too willing to play upon our recently heightened awareness of immune systems. I hope you'll be skeptical of products that promise to boost any immune systems — whether your own or your fishes' — no matter how "natural" the ingredients are made to appear.
 
Spirulina, for an example, is sometimes the subject of immune-system-boosting claims. Perhaps it is so. But recall that there is virtually no government oversight concerning what kind of claims are legitimate. When the Federal Food and Drug Administration considered regulating "health food" supplements in the early 1990s, big corporate guns were brought to bear, and the upshot was that a new category, termed "dietary supplements," was created, subject neither to the regulations that govern drugs nor to the manufacturing standards applied to human foods. So the marketing of human "dietary supplements" has been provided an untramelled arena. On another front, the pet supplies industry itself is all but unregulated. You can judge for yourself whether possible abuses of production and labeling of those "dietary supplements" that are meant for pet animals, which are not intended for eventual human consumption, are any less rampant.