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.

Spring viraemia of carp, also known as Swim Bladder Inflammation, is caused by a rhabdovirus called "Rhabdovirus carpio". It is listed as a notifiable disease under the World Organisation for Animal Health.

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.

The main symptoms are diarrhea and colicky abdominal pain. Because symptoms are often mild, infections can often be easily overlooked but diagnosis is important. Flukes attach to the wall of the small intestine, but are often asymptomatic unless in large numbers. Infection can occur from eating a single infected fish source. Peripheral eosinophilia is associated especially in early phase. When present in large numbers, can cause chronic intermittent diarrhea, nausea, and vague abdominal pains. Clinical complaints can also include lethargy and anorexia. In acute metagonimiasis, clinical manifestations are developed only 5–7 days after infection. Heavy infection has also been associated with epigastric distress, fatigue, and malaise.

Occasionally, flukes invade the mucosa and eggs deposited in tissue may gain access to circulation. This can then lead to eggs embolizing in the brain, spinal cord, or heart. Granulomas may form around eggs and can cause seizures, neurologic deficits, or cardiac insufficiency.

An interesting case in Japan found Diabetes Mellitus (DM) to be a sign of chronic infection with intracerebral hemorrhages as the acute sign of aggravation. Two months after administering the appropriate drug, Praziquantel, the ICHs were gone, as was the man's Diabetes Mellitus. This unique case shows the potential of additional symptoms associated with metagonimiasis that are still unknown.

Metagonimiasis is a disease caused by an intestinal trematode, most commonly "Metagonimus yokagawai", but sometimes by "M. takashii" or "M. miyatai". The metagonimiasis-causing flukes are one of two minute flukes called the heterophyids. Metagonimiasis was described by Katsurasa in 1911–1913 when he first observed eggs of "M. yokagawai" in feces (date is disputed in various studies). "M. takahashii" was described later first by Suzuki in 1930 and then "M. Miyatai" was described in 1984 by Saito.

Stained adult fluke causing metagonimiasis

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.

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"

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.

The olfactory system is the system related to the sense of smell (olfaction). Many fish activities are dependent on olfaction, such as: mating, discriminating kin, avoiding predators, locating food, contaminant avoidance, imprinting and homing. These activities are referred to as “olfactory-mediated.” Impairment of the olfactory system threatens survival and has been used as an ecologically relevant sub-lethal toxicological endpoint for fish within studies. Olfactory information is received by sensory neurons, like the olfactory nerve, that are in a covered cavity separated from the aquatic environment by mucus. Since they are in almost direct contact with the surrounding environment, these neurons are vulnerable to environmental changes. Fish can detect natural chemical cues in aquatic environments at concentrations as low as parts per billion (ppb) or parts per trillion (ppt).

Studies have shown that exposures to metals, pesticides, or surfactants can disrupt fish olfaction, which can impact their survival and reproductive success. Many studies have indicated copper as a source of olfactory toxicity in fishes, among other common substances. Olfactory toxicity can occur by multiple, complex Modes of Toxic Action.

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".

Early investigation by Hasler and Wisby (1951) examined how fish use olfactory imprinting to discriminate smells in order for fish to find their natal streams. This research provided the framework for testing synthetic chemicals used by hatcheries to examine homing and straying by hatchery fish. The investigation of the toxicity of mercury and copper to the olfactory systems in fish began in the early 1970s. Where they found that solutions of mercury chloride (HgCl) and copper sulfate (CuSO) depressed olfactory response during exposure to the two toxicants and found that toxicant concentration and olfactory response had an inverse relationship to each other.