If you have been born a healthy baby, you have a good chance of reaching your fifth birthday. A very good chance, actually: only 1 in 26 children don't make it to this age. The two biggest killers of children under 5 are pneumonia and diarrhoea, which at first sight have little in common; yet epidemiological studies have shown that children suffering from diarrhoea seem to be oddly more at risk of catching pneumonia later. We know it happens, but not necessarily why.
A recent study has investigated this mechanism in mice: those infected with bacteria causing intestinal infections (Salmonella typhimurium) had a much higher chance of having pneumonia-causing bacteria (Klebsiella pneumoniae) in their lungs and being sick a couple of days later.
The study suggests a possible mechanism: the gut infection seems to be impairing the lung neutrophils (the most common type of white cells, which fight infections), which are less able to clear the lung bacteria. This is a potential way the "gut-lung immunological crosstalk" is working, although we still don't understand it very well.
In summary, getting sick in your gut can make your lungs sick. Another reason to prevent diarrhoea, if we needed one!
More information: Intestinal infection results in impaired lung innate immunity to secondary respiratory infection
Spotted by: Amy Pickering