Diseases that can be spread are Infectious.
This means that when one person has a disease, they may pass that disease on to someone else. Or, another organism may be responsible for passing on an infectious disease (see below).
Infectious disease can be caused by a bacterium (a prokaryote) or single-celled organism (a karyote) or virus. Even fungi can be responsible for causing illness (e.g. ringworm is caused by a fungal organism and it's highly infectious).
Useful advice: Make sure you know the structure of a typical bacterial cell. Be able to label the cell wall, the slime layer or capsule, the pilus (pl. pili), the chromosome or nucleoid, ribosomes, plasma membrane, flagellum. Useful link: http://homepages.uel.ac.uk/u0220158/
All these disease-causing agents are sometimes carried by a host organism known as the "vector". The vector could be an insect and a prime example of an insect-transmitted disease is malaria.
Malaria is caused by a protozoan (a karyoyte) named Plasmodium vivax. It is one of the most important of human pathogens causing over 1 million deaths every year.
P. vivax needs the mosquito and humans to complete its lifecycle. When an infected mosquito bites a human (only the female mosquito carries the parasite), it passes on sporozoites which are small cells of the parasite.
The sporozoites replicate in the human liver and eventually enter the blood stream to infect red blood cells.
Infected red blood cells cannot function properly to transport respiratory gases and are in effect lost as carriers of oxygen and carbon dioxide (see image of affected red blood cells). Anaemia therefore accompanies malaria plus, enlargement of the spleen.
There is no really effective method of controlling the malarial mosquito which is present in most parts of the world where the climate is hot and there are plenty of areas of standing water. This is where the mosquito breeds.
