Insects transmit parasites in various manners.
This may be because the gut becomes blocked, as in plague-infected fleas, or because the parasite migrates back up the gut and out through the mouth, as in the case of the parasites that cause sleeping sickness and leishmania. These parasites pass out of the gut into the body cavity and then back out through the mouth parts again. All of these parasites pass into the body cavity through the insect's gut. Malaria parasites enter the mosquito through the digestive tract and exit by invading the salivary glands. Worms causing filariasis do not re-enter the digestive tract but break through the cuticle of the mouth parts while the insect is taking a blood meal.