Spring viraemia of carp virus has been shown to infect a wide variety of fish species including silver carp, grass carp, crucian carp, and bighead carp. It has also been shown experimentally to infect other fish species including northern pike, guppies, zebrafish, and pumpkinseed. It is considered to be a major threat to naive fish populations, especially farmed fish including ornamental koi and common carp.

Clinical symptoms of viral infection include external hemorrhaging, pale gills, and ascites. In some cases, mortality can occur without any apparent clinical signs of the disease. The virus has been found in high concentrations in the liver and kidney, but lower numbers of virions have been isolated from the spleen. The virus has been shown to persist subclinically in fish populations up to 10 weeks following experimental infection. Currently efforts have been made to prevent infection by the virus through the development of DNA vaccines and immunostimulatory therapeutics.

All fish carry pathogens and parasites. Usually this is at some cost to the fish. If the cost is sufficiently high, then the impacts can be characterised as a disease. However disease in fish is not understood well. What is known about fish disease often relates to aquaria fish, and more recently, to farmed fish.

Disease is a prime agent affecting fish mortality, especially when fish are young. Fish can limit the impacts of pathogens and parasites with behavioural or biochemical means, and such fish have reproductive advantages. Interacting factors result in low grade infection becoming fatal diseases. In particular, things that causes stress, such as natural droughts or pollution or predators, can precipitate outbreak of disease.

Disease can also be particularly problematic when pathogens and parasites carried by introduced species affect native species. An introduced species may find invading easier if potential predators and competitors have been decimated by disease.

Pathogens which can cause fish diseases comprise:

- viral infections, such as esocid lymphosarcoma found in "Esox" species.

- bacterial infections, such as "Pseudomonas fluorescens" leading to fin rot and fish dropsy

- fungal infections

- water mould infections, such as "Saprolegnia" sp.

- metazoan parasites, such as copepods

- unicellular parasites, such as "Ichthyophthirius multifiliis" leading to ich

- Certain parasites like Helminths for example "Eustrongylides"

Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens.

Specific defences are specialised responses to particular pathogens recognised by the fish's body, that is adaptative immune responses. In recent years, vaccines have become widely used in aquaculture and ornamental fish, for example vaccines for furunculosis in farmed salmon and koi herpes virus in koi.

Some commercially important fish diseases are VHS, ich and whirling disease.

Metagonimiasis infections are endemic or potentially endemic in 19 countries including Japan, Korea, China, Taiwan, the Balkans, Spain, Indonesia, the Philippines and Russia. Human infections outside endemic areas may result from ingesting pickled fish or sushi made from fish imported from endemic areas.

Several public health prevention strategies could help lower the rates of metagonimiasis. One is to control the intermediate host (snails). This can be done through use of molluscidals. Another is to use education to ensure all people, especially in areas were the disease regularly occurs, fully cook all fish. This could potentially be problematic and not as effective as hoped as many of the people affected by metagonimiasis eat raw or pickled fish as part of a traditional, long-seated dietary practice. Additionally, implementing more sanitary water conditions would reduce the continual reintroduction of eggs to water sources, thus restarting the lifecycle. Complete control of metagonimiasis presents several potential problems because it does have several reservoir hosts, thus eradication is unlikely.

Confluent and reticulated papillomatosis of Gougerot and Carteaud (also known as "Confluent and reticulated papillomatosis," "CRP", "CARP", "Familial cutaneous papillomatosis," and "Familial occurrence of confluent and reticulated papillomatosis") is an uncommon but distinctive acquired ichthyosiform dermatosis characterized by persistent dark, scaly, papules and plaques that tend to be localized predominantly on the central trunk.

The disruption of olfaction and potential effects to survival and reproductive success at environmentally-relevant concentrations metals, pesticides or surfactants have implications for fish and salmon recovery because these are commonly found in western United States streams. Conventional, acute and chronic toxicity testing do not explicitly address nervous system function and underestimate thresholds for toxicity in salmonids. Since these effects are not explicitly looked at during studies they oftentimes can go unnoticed. Olfactory toxicity occurring at environmentally relevant concentrations can induce reduction to food odor attraction and predator scent or alarm response pheromones can cause major problems with survivorship. Olfactory toxicity can also affect the ability of anadromous fish to find their natal stream causing them to stray to other streams.

Henri Gougerot and Alexandre Carteaud originally described the condition in 1927. The cause remains unknown, but the observation that the condition may clear with Minocycline turned attention to an infectious agent. "Actinomycete Dietzia" strain X was isolated from one individual. Other antibiotics found useful include azithromycin, fusidic acid, clarithromycin, erythromycin, tetracycline and cefdinir.

Avoidance behavior exhibited by fish is species specific, Whitefish ("C. clupeaformis") showed a preference toward SLS at a concentration of 0.1 mg/L while rainbow trout ("Oncorhynchus mykiss") and common carp ("Cyprinus carpio") showed an avoidance response at a concentration of 0.01 ug/L. Past studies are difficult to compare due to differences in test and exposure conditions.

Polyploid cells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (Eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. This is known as endopolyploidy. Species whose cells do not have nuclei, that is, Prokaryotes, may be polyploid organisms, as seen in the large bacterium "Epulopiscium fishelsoni" . Hence ploidy is defined with respect to a cell. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Male bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.

Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Therefore, the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes.

Polyploidy may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will also double the existing chromosome content.

Polyploidy occurs in highly differentiated human tissues in the liver, heart muscle and bone marrow. It occurs in the somatic cells of some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see "Hibiscus rosa-sinensis"), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus "Brassica" are also tetraploids.

Polyploidization is a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant "Erythranthe peregrina". Sequencing confirmed that this species originated from "E. x robertsii", a sterile triploid hybrid between "E. guttata" and "E. lutea," both of which have been introduced and naturalised in the United Kingdom. New populations of "E. peregrina" arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of "E. x robertsii". Because of a rare genetic mutation, "E. peregrina" is not sterile.

Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter "x" is used to represent the number of chromosomes in a single set.

- triploid (three sets; 3"x"), for example seedless watermelons, common in the phylum Tardigrada

- tetraploid (four sets; 4"x"), for example Salmonidae fish, the cotton "Gossypium hirsutum "

- pentaploid (five sets; 5"x"), for example Kenai Birch ("Betula papyrifera" var. "kenaica")

- hexaploid (six sets; 6"x"), for example wheat, kiwifruit

- heptaploid or septaploid (seven sets; 7"x")

- octaploid or octoploid, (eight sets; 8"x"), for example "Acipenser" (genus of sturgeon fish), dahlias

- decaploid (ten sets; 10"x"), for example certain strawberries

- dodecaploid (twelve sets; 12"x"), for example the plants "Celosia argentea" and "Spartina anglica" or the amphibian "Xenopus ruwenzoriensis".