Viruses and Disease

Viruses cause disease in several ways, and the symptoms of an infection come from a mix of damage done by the virus and the body's own response.Destroying host cells. When viruses replicate by the lytic pathway, infected cells burst and die. If enough cells are lost, the tissue stops working. Influenza destroys the epithelial cells lining the airways, and poliovirus kills motor neurones, which can cause paralysis.Disrupting cell function. Even without bursting cells, a virus takes over the cell's ribosomes, nucleotides and ATP, so the cell cannot make its own proteins or carry out its normal job.The immune response. Many symptoms are caused by the body fighting the virus. Fever, inflammation, a runny nose and aching muscles come from chemicals released by immune cells. Cytotoxic T cells also kill infected body cells.Weakening the immune system. HIV infects and destroys T helper cells. As their number falls, the body cannot fight other pathogens, and the person develops AIDS, in which opportunistic infections such as tuberculosis and pneumonia become life-threatening.Causing cancer. Some viruses change the control of the cell cycle in the cells they infect. Human papillomavirus (HPV) can cause cervical cancer, and hepatitis B virus can cause liver cancer.Examples of viral diseases in humans include influenza, measles, chickenpox, the common cold, COVID-19, hepatitis, HIV/AIDS and Ebola. Viruses also cause disease in plants (tobacco mosaic virus damages chloroplasts, giving a mottled leaf) and in farm animals (foot-and-mouth disease).