Gnathostomiasis infections are rare worldwide. They're reported mostly in Southeast Asia, but are now also seen in Central and South America. This type of infection is considered by the Centers for Disease Control and Prevention (CDC) to be an emerging imported disease. This means that it's spreading around the world just as we do as travelers.
Gnathostomiasis is caused by gnathostoma spinigerum, a helminth-type parasite, which you are at risk for ingesting when you eat undercooked or raw infected foods, including freshwater fish, crustaceans (shrimp, crab, crayfish), frog meat, pork and chicken.
Humans aren't actually very good hosts for this parasite, which can live for as long as a decade in your body after you've been infected. We're unintended, accidental hosts for this organism -- they aren't able to reproduce inside a human body as they do inside other hosts. Good for you, but bad for the parasite. Instead, they live out their 10 to 12 year life cycle migrating throughout your body, causing swelling under your skin.