We've all experienced disease firsthand in some way--whether as the common cold or the flu, or in much more serious forms. Although medicines are available for many diseases, God has also mercifully provided our bodies with means of fighting off disease from the inside. We are designed with an immune system to combat pathogens--tiny disease-causing organisms such as bacteria, protozoans, molds, or viruses.
Diseases, which are any condition of the body which interfere with its normal functioning, fall into two categories: infectious and noninfectious. Allergies, congenital or inherited diseases like hemophilia, hormonal diseases like diabetes, and nutritional and psychosomatic diseases are all noninfectious--they are caused by breakdown in the body's systems or functions. Infectious diseases include chicken pox, cold sores, influenza, HIV, and venereal diseases, and are caused by harmful invading microbes. This type of disease is communicable--it can be transferred from one person to another through germs.
Our bodies are provided with protective coatings, chemicals, and filtering systems to protect us from disease. Mucous membranes coat the inside of our noses, trapping alien particles like dust so that they don't enter our bodies. Lysozymes in tears and perspiration kill bacteria by destroying their cell walls, and hydrochloric acid in the stomach kills most of the bacteria that enters it.
Our lymphatic systems keep our cells in the proper fluid balance, absorb fat, and provide immunological defense. Bathing our cells is an interstitial fluid, a clear liquid that flows from the capillaries in our circulatory systems; most of the 'used' fluid is drawn back into the bloodstream and recycled, but about 10% of it goes into the lymphatic system. If we weren't provided with a place for this 10%, our bodies would suffer swelling from the excess fluid.
Once the interstitial fluid enters the lymph vessels, it is called lymph. There is no heart to pump fluid through the lymphatic system like there is in the circulatory system, but the contraction of skeletal muscles propels the lymph through the vessels. Lymph nodes, points along the vessels at which the lymph is cleaned, contain lots of white blood cells, or lymphocytes. These special cells can destroy harmful microbes by phagocytosis--they engulf a microbe and then break it down. Another part of this system are the larger 'cleaning' centers, the lymph organs--the tonsils, spleen (although it filters blood, not lymph), and thymus among them. After the lymph has been carried through the vessels and filtered, it is emptied back into the bloodstream.
Our bodies usually are very effective at self-defense, but sometimes they get diseases which cause the immune system to go haywire. Rheumatoid arthritis is a condition where the body attacks its own tissues, and AIDS is a disease that impairs the functioning of the immune system, so that the body no longer is capable of fighting off other diseases effectively. Also, sometimes our bodies need outside help to effectively defeat an infection--this is where medicine is helpful. It might be useful to think of medicine as the reinforcing army regiment sent to aid or relieve the regiment that has been on the front lines (the immune system), fighting a force that is too superior. Medicines are designed to kill an infection without damaging the body's normal cells.