This chapter describes various causes of communicable diseases, such as, bacteria, chlamydia, fungi, parasites, rickettsiae, and viruses. It describes the important link between tuberculosis and human immunodeficiency virus (HIV). The chapter provides the five species of Plasmodium that cause malaria: plasmodium falciparum, plasmodium vivax, plasmodium ovale, plasmodium malariae, and plasmodium knowlesi. It also describes schistosomiasis and Chagas disease. The chapter explains the symptoms of trypanosomiasis. It then identifies the risk factors for cardiovascular disorders and provides the risk factors for cancer. It also differentiates the most common types of diabetes: Type 1 diabetes, and Type 2 diabetes. The chapter then outlines the four major mental health disorders throughout the world. They are, depression, anxiety disorders, schizophrenia, and bipolar disorder. It then describes fetal alcohol syndrome. Influenza is a viral infection of respiratory tract. These viruses pose a serious public health problem since severe illness and death can occur. The most effective way to prevent infection is with an influenza vaccination.