## 3 Bacterial, Viral, and Parasitic Diseases in Fish  
Fish are susceptible to a variety of diseases, both infectious and non-infectious. Infectious diseases are caused by a pathogenic organism, either encountered in the environment or spread through contact. Non-infectious diseases are caused by a combination of environmental and genetic anomalies and are not contagious. Infectious diseases are transmitted to fish either directly or indirectly and are expediated by external conditions such as poor water quality and contaminated culture. There are three types of infections that fish commonly suffer from: parasitic, bacterial, and viral. The most common bacterial, viral and parasitic diseases and their current vaccination status are listed in Table 1.  
Fish are commonly infected by pathogenic bacterial species belonging to the genera _Edwardsiella_, _Vibrio_, _Aeromonas_, _Streptococcus_, and _Flavobacterium_[38]. Bacterial pathogens are ubiquitous in the aquatic environment [39]. Induced stress and causative factors (e.g., organic pollution) can prompt a bacterial infection outbreak [40]. Symptoms of bacterial infection are often exhibited through exophthalmia, epithelial lesions and anorexia, all of which can be treated with antibiotics and vaccination. Intraperitoneal or bath vaccination are favored to treat bacterial infections, but in severe cases, antibiotics are administered [1]. There are existing vaccinations for several, but not all, bacterial infections. Virus-host interactions can cause alterations at the cellular level and the viral infection can spread rapidly through manipulation of host DNA. Well-characterized DNA viruses include _Iridoviridae_, _Herpesviridae_, and _Adenoviridae_[41]. However, some organisms are  
Figure 1: Once infected cells have sensed an invading pathogen, the fish immune system responds through a cell-mediated and a humoral response. The cell-mediated response involves the replication of cytotoxic cells, which lyse infected cells, and T-helper cells, which produce cytokines that aid in moderating the overall immune response. The humoral response involves the replication of plasma cells, which produce antibodies against the pathogen, and memory B cells, which retain information regarding the pathogen, ensuring a quick response if the pathogen invades again. able to replicate inside the host organism through the insertion of RNA contents. RNA viruses are represented in the aquaculture world in the form of _Retroviridae_, _Rhabdoviridae_, _Paramyxoviridae_, and _Orthomyxoviridae_[41]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name of the Diseases** & **Causative Agent** & **Fish It Affects** \\ \hline \multicolumn{3}{c}{Bacterial} \\ \hline \multicolumn{3}{c}{Atypical furunculosis} & _Aeromonas salmonicida_ & Salmonids, spotted wolfish, Atlantic cod \\ \hline \multicolumn{3}{c}{Motile aeromonid septicemia} & _Aeromonas hydrophilla_, _A. caviae_, & Freshwater fish species, including catfish \\ \multicolumn{3}{c}{_A. veroniti bivor sboria_} & and brass \\ \hline \multicolumn{3}{c}{Vibrio spp., including _V. harveyi_, _V._} \\ \multicolumn{3}{c}{Vibriosis} & _vulnificus_, _V. alginolyticus_, and & Marine fish including salmonids, yellowtail, \\ \multicolumn{3}{c}{_V. parahaemolyticus_} & halibut, amberjack \\ \hline \multicolumn{3}{c}{Enteric septicemia} & _Edwardsiella itcaluri_ & Catfish \\ \hline \multicolumn{3}{c}{Edwardsiellosis} & _Edwardsiella tarda_ & Catfish, striped bass, tilapia, sea bream \\ \hline \multicolumn{3}{c}{Tuberculosis} & _Mycobacterium marinum_, _M.