An acute upper respiratory tract infection is a short-term bacterial or viral infection. The common cold is an example of a viral acute upper respiratory infection. Other causes of upper respiratory infections include bacteria, environmental irritants (such as airborne pollution or tobacco smoke), and allergies.
The virus/bacteria/environmental irritants can enter your respiratory system via the following ways:
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Breathing in air containing the virus/bacteria/environmental irritants - an infected person sneezes or coughs, causing the virus/bacteria to become airborne
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Touching an infected surface or shake hands with a person who’s sick, then touching your mouth, nose or eyes