A disease causes malfunction of body systems and has specific signs and symptoms. Diseases may be caused by external causes in the case of infectious diseases or can be caused by internal malfunctions or by autoimmune disease, where the body attacks its own organs. Pathogenic microbial agents that cause infectious disease include bacteria, fungi, protozoa, parasitic worms, protozoa and prions. Diseases may be caused by cancer, genetic disorders and cardiovascular disease. A disease can be a result of a vitamin deficiency, for example, the disease scurvy is due to lack of vitamin C. In the case of genetic disorders, if a gene has a mutation, the cells will produce a mutant protein. If this protein is partially functional or not functional at all, a disease can result.An example is sickle cell anemia, a disease caused by a mutation in the hemoglobin gene and the resulting hemoglobin folds in a shape that is unable to adequately pass through the blood vessels, resulting in less oxygen being transported to the cells and disease symptoms.