Describe the disease transmission cycle

The transmission of pathogens from current to future host follows a repeating cycle. This cycle can be simple, with a direct transmission from current to future host, or complex, where transmission occurs through (multiple) intermediate hosts or vectors.

T.b. gambiense sleeping sickness is transmitted from human to human by the tsetse fly which is the most common form of transmission. As some animals can host the human pathogenic parasite, transmission can occasionally take place directly from animals to humans, which is believed to be one of the causes of the long term maintenance of the disease in endemic areas. However the epidemiological importance of such zoonotic transmission is poorly understood.

Ithink is b. or c.