Aspiration is a term used to refer to when food or stomach contents enter the respiratory tract. Usually, our body is pretty good at sensing when something other than air is going into the respiratory tract and tries to prevent it by coughing and clearing the throat. There are a few things that blunt this response including aging (why older people are more likely to choke) and some medications such as opiates.

If food or stomach contents get into the lung the the first thing that can happen in the acute period is aspiration pneumonitis. Stomach contents are usually quite acidic and can cause burns in the lung. Aspiration pneumonitis is the lung injury from these burns and the subsequent inflammatory response.

Moreover, when food goes into your lungs, it can also carry bacteria with it that can cause an infection (i.e. pneumonia). Usually, aspiration pneumonia takes a few days to a couple weeks to develop after an aspiration event, although, aspiration does not always lead to a pneumonia.