Antibiotics provide no benefit for the common cold and other respiratory illnesses caused by viruses. Estrada Anton/Shutterstock

“When you have a cold or other respiratory illness, you might see a range of different colours of mucus or snot when you blow your nose. We’re often told – even by doctors – that green or yellow secretions indicate you’re infectious. But this isn’t true.

“It’s unclear how this myth arose, but it’s likely a misunderstanding of the appearance and colour of pus.

“Pus usually signals the arrival of harmful bacteria to a site, such as golden staph into a hair follicle resulting in a boil. But in the case of respiratory infections, the green or yellow colour is due to the white blood cells.

“During a viral infection, the lining of the respiratory cells is damaged, … ”

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