Can bacteremia remain in the bloodstream for quite a while until one day the immune system becomes weaker and then cause sepsis?

No: Bacteria do not remain dormant in blood. However, we all get transient invasion of blood with bacteria from activities like brushing teeth and having bowel movement. In a person with weak immune system, such, otherwise benign, bacteremias can cause sepsis. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. 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, if you have sex.