Pneumonia is an acute infection of the parenchyma of the lung caused by bacteria, fungi, virus, parasite etc. Pneumonia usually starts when you breathe the germs into your lungs. You may be more likely to get the disease after having a cold or the flu. These illnesses make it hard for your lungs to fight infection, so it is easier to get pneumonia. Having a long-term or chronic, disease like asthma, heart disease, cancer or diabetes also makes you more likely to get pneumonia.