The document discusses the immune system and provides definitions of key immune system terms like macrophage, T-cell, and thymus gland. It also defines vaccines. The document then focuses on influenza (flu) as a disease, noting the flu attacks the lungs, nose and throat. It especially affects young children, older adults, pregnant women, and those with chronic diseases or weak immune systems. Symptoms of the flu include fever, chills, muscle aches, cough, congestion, runny nose, headaches and fatigue. The document lists home remedies and over-the-counter medications to cope with flu symptoms and mentions prevention through nasal spray and flu shots.