Hello .. Welcome to Health Care Magic.. I read your query and it seems that repeated exposure to Salmonella typhi is responsible for your mothers typhoid posite results.. I sugest her to be on antibiotics even after symptoms of fever is off , I would recommend Ciprofloxacin as antibiotic of choice which has good effect in treatment of typhoid fever . Ty21a vaccine is administered intramuscularly and it is recommended to take a booster shot after five years. Typhoid is a completely avoidable condition, avoid locally prepared foods and water to keep the disease away . (Pakaged or boiled water is safest) . Hands should be clean before cooking or eating . Do not eat fruits and raw vegetables that have been precut. Provide plenty of water to your mother as it prevent the dehydration that results from a prolonged fever and weakness. She may need to receive fluids through a vein in your arm (intravenously) if severely dehydrated. Hope this information is helpful . Thanks & Take Care .