One disease can make another more likely or more serious. HIV is a virus that infects and destroys white blood cells in the immune system. As these cells are lost, the body cannot fight off pathogens.
People with HIV are much more likely to develop tuberculosis (TB), a bacterial disease of the lungs, and it is usually more severe in them. A person can carry TB bacteria without symptoms. When HIV weakens the immune system, the bacteria can become active.
A communicable disease can also lead to a non-communicable one. Human papillomavirus (HPV) is a virus spread by sexual contact. Some types of HPV cause changes in cells of the cervix, which may develop into cervical cancer. A vaccine against HPV lowers the risk of cervical cancer.