There's flu and then there's complications from flu. This one is a pneumonia complicated by pus formation. Think of a 2 liter soda bottle, empty, and then put a balloon in it and blow it up so that the balloon takes the shape of inside the bottle. The bottle is your chest wall and the balloon is your lung, filled with air. Put water in the balloon and you have pneumonia. But put an inch or two of water in the bottle, and then put the balloon in, and you get water outside the balloon/lung but inside the bottle/chest. It's a complication of pneumonia called a parapneumonic effusion and it's not good. It often has to be drained via a hole in the bottle/chest wall. But if instead of fluid you get pus, it's called empyema. That's even worse, and is difficult to treat, especially in young children (i.e. it's hospital stuff with maybe an invasive procedure for drainage. Do not try this at home.)
I include this because people often say, "but 1918 was primitive, and we have fancy medicine." Nuh-uh. Even modern medicine is seeing increasing bacterial resistance and virulence (think MRSA, another potential flu complication). If hospitals are full, medications are in short supply, and you have to deal with this at home, you are in big trouble. And if you want to add in the health reform/finance/insurance issues that interfere with excellent and timely care... well, in the meantime, get your flu shot.
Bacterial pneumonia with empyema is a serious complication of influenza and commonly resulted in death during the 1918 influenza pandemic. We hypothesize that deaths caused by parapneumonic empyema are increasing in Utah once again despite advances in critical care and the availability of antimicrobial drugs and new vaccines. In this study, we analyzed the historical relationship between deaths caused by empyema and influenza pandemics by using 100 years of data from Utah. Deaths caused by empyema have indeed increased from 2000–2004 when compared with the historic low death rates of 1950–1975. Vaccine strategies and antimicrobial drug stockpiling to control empyema will be important as we prepare for the next influenza pandemic.