Relationships between myxosporeans and their hosts are often highly evolved and do not usually result in severe diseases of the natural host. Infection in fish hosts can be extremely long-lasting, potentially persisting for the lifetime of the host. However, an increasing number of myxosporeans have become commercially important pathogens of fish, largely as a result of aquaculture bringing new species into contact with myxosporeans to which they had not been previously exposed and to which they are highly susceptible. The economic impact of such parasites can be severe, especially where prevalence rates are high; they may also have a severe impact on wild fish stocks.