In 2004, there was an influenza vaccine crisis in the United States, when many lots of vaccine had to be recalled. The problem was bacterial contamination. Why would this be a problem with an...

In 2004, there was an influenza vaccine crisis in the United States, when many lots of vaccine had to be recalled. The problem was bacterial contamination. Why would this be a problem with an injectable vaccine? Would it have been a problem with an oral vaccine? If so, under what conditions? There was only one major manufacturer of the vaccine, a fact that made the shortage worse. How could this shortage of manufacturers have happened?