What is the agent responsible for causing the following diseases, and what measures can be taken to prevent them?

a) Malaria

b) Cholera

a) Malaria is a parasitic disease that is caused by a type of protozoan parasite of the genus Plasmodium. It is transmitted to humans through the bite of infected female Anopheles mosquitoes. 

To prevent malaria, measures such as using insect repellents, and sleeping under mosquito nets are recommended. Additionally, efforts to control mosquito populations through insecticide spraying and eliminating breeding sites can help reduce transmission.

b) Cholera is caused by the bacterium Vibrio cholerae, typically spread through contaminated water and food. 

Prevention include ensuring access to safe drinking water, practising good hygiene such as regular handwashing with soap and water, properly cooking food, and avoiding raw or undercooked seafood, particularly in areas where cholera is endemic. Additionally, vaccination against cholera can be recommended in high-risk areas or for travellers visiting regions where cholera outbreaks occur.