Parasites are organisms -- usually very small ones -- that feed on a larger organism. Bettas are susceptible to a number of parasites that without prompt medical treatment can kill them. With excellent husbandry, however, you can greatly reduce your betta's risk of getting a parasite.
Because parasites are living organisms, they never come out of nowhere. Other fish are often the culprits when a betta gets infected with a parasite. Putting your betta with fish such as goldfish, cichlids or saltwater fish who live in much different natural environments greatly increases his risk of developing a parasite. If you house your betta with other fish, quarantine any new fish for a week before adding them to the tank, and never place bettas with fish who have much different ecological needs.
Many store-bought bettas already have parasites. Pet stores may not provide proper care for the fish, may expose bettas to other fish who have parasites or may buy from unscrupulous breeders who breed unhealthy fish. Because most people buy their bettas from pet stores, it's important to quarantine your fish for several days if you plan to house him with other fish. Otherwise he may transmit parasites to all of your fish.
Poor water quality is a significant factor in the development of parasites. When water isn't cleaned enough, this provides a breeding ground for tiny organisms. Water that is too hot or too cold can also make it more likely that your tank will develop parasites your betta's immune system is not equipped to fight off. Change the water frequently and provide ample filtration. Water pH can also cause problems with parasites. Bettas prefer water with a neutral pH of 7 or slightly below.
When bettas are in poor health, they're more susceptible to parasites and the parasites are more likely to kill them. If your betta stops eating, becomes lethargic or otherwise seems sick, consult a fish veterinarian or a fish expert who can help you diagnose the condition. By treating illness early and aggressively, you can save your betta and prevent the development of life-threatening parasites.