Dr. Ross knoew that the disease of malaria was caused by an organism called plasmodium.  The Plasmodium was found in the blood stream of infected individuals but it was not known how it got there.  Dr. Ross also knew that people who slept in tents that were not protected were more likely to contract malaria that those with protected sleeping envrironments.  Int ehe unprotected sleeping areas he found large amounts of mosquitos.  Dr. Ross hypothesized that if the mosquito was spreading malaris, mosquitoes that had bitten infected patients would pick up the plasmodium parasite. 
Dr. Ross found that mosquitos that carried the disease also carried the plasmodium.  His experiment was to sample the blood of individuals known not to have malaria and the cause them to sleep in mosquito netted areas of a hospital.  He also took another group known to have malaria and cause them to sleep in unprotected areas of a hospital.  Dr. Ross then sample the mosquitos that fed on the malaria patients and they were found to have the plasmodium in their gut.