Disease affects an organism by disrupting the organism's homeostasis. Homeostasis is the process by which an organism maintains a steady internal environment (stable body temperature, blood sugar level, electrolyte balance, etc.). A disease will disrupt this process. For example, Helicobacter pylori (the bacteria responsible for most stomach ulcers in humans) disrupts homeostasis by interfering with the production of the mucus layer that protects the stomach lining cells from the hydrochloric acid in the stomach lumen.
Bacteria alike spores come in hand to hand; meaning one can cause the other if not treated properly and in time. Perhaps washing hands with soap for 20 seconds, when coughing cover cough into your elbow and avoid sneezing into the open air; use tissue to hold your sneeze.
Human activity can affect the amount of carbon in the atmosphere. When fossil fuels like oil, coal, and natural gas are burned, carbon dioxide is released into the atmosphere as waste.