I don't know about other cultures , but in the Caribbean we drink teas to boost our immune system. Growing up in Jamaica with my grandmother and my mother, They brewed a variety of natural tea leaves ,in order to ensure that my sister and I begin each day ,with a warm stomach. The famous 'Soursop Leaf Tea' was apart of the tradition. We had about three soursop trees next to our little, sand dashed, board house. Although my mother and grandmother are both deceased, I can still hear their voices in my head ." Poochie ,go and pick some Soursop leaves, wash them and bring them come to me." Man , I tell you! Although this tea is not as famous as Chamomile, Lavender or Green tea, it is known as a champion in the Caribbean, where it is used as a folk remedy/ medicine. We usually drink it to keep sickness at bay and to fight bacteria or viruses from within.
This tea has been rumored to assist in building the immune system, kill cancer cells during the early phase of cancer, relax your nerve ,lower blood pressure, and when combined with cinnamon it helps to lower your blood sugar.
Although Soursop as been used in folk medicine ,often with encouraging results for many years and people boast about it results, please use it as a supplement and not a a replacement for your usual medication.