How do your white blood cells initially find out which cells have bacteria or viruses in them? do they randomly check all your body cells?

Complicated issue: White blood cells routinely monitor body cells and if they have any proteins that are not natural part of the body of the person, the white cells can recognize those as foreign and react against the cell expressing the foreign material. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.