Pneumonia is a lung infection causing inflammation in the air sacs (alveoli), filling them with fluid or pus, leading to symptoms like cough (with phlegm/pus), fever, chills, and difficulty breathing. It's caused by germs (bacteria, viruses, fungi) and can range from mild to life-threatening, affecting anyone but posing higher risks for the very young, elderly, and those with weakened immune systems. Treatment involves antibiotics (for bacterial), antivirals, rest, fluids, and sometimes oxygen, with recovery varying by severity.