The key to a healthy body is having a strong immune system. An immune system is a complex system that, when functioning properly, fights bacteria, viruses, fungi, and parasites. Good nutrition is essential for developing and keeping the immune system healthy and strong. Critical nutrients that stimulate a strong immune system include vitamins A, C, E and essential fatty acids. The most important minerals required include manganese, selenium, zinc, copper, iron, sulphur, and magnesium.