The catarrhal phase refers to the initial stage of certain infectious diseases, particularly viral infections like influenza or whooping cough, characterised by inflammation of mucous membranes, leading to symptoms such as nasal congestion, sneezing, and a dry cough. Recognising this phase is crucial for early diagnosis and management, as it can help prevent complications like secondary bacterial infections. An example is the catarrhal phase of whooping cough, which can progress to severe coughing fits.