The main ‘vulnerable’ groups include the very young and very old age groups, which may not affect you as an employer, but they also include pregnant women, which does affect you as an employer. These vulnerable groups will have weaker immune systems and hence they are more susceptible to the flu virus. The flu virus itself also weakens the immune system so people are more exposed to secondary bacterial infections such as pneumonia. And people with conditions such as heart problems, chest complaints, kidney disease, liver disease, stroke recovery and diabetes can experience severe complications if they also contract flu, so they are classed as vulnerable too.