To characterise the physical parameters (number concentration, size distribution and their trajectories) of aerosols (droplets) generated in high risk procedures.

Inclusion Criteria:

1. Adult patients (>18 years old) that can give valid consent
2. URTI: with symptoms such as coryza, sore throat, nasal discharge, cough and sputum +/- fever. Amongst these, cough should be a major complaint.
3. LRTI (CAP): acute lower respiratory illness of no other known cause which is usually associated with fever, symptoms and signs of the chest and abnormalities on the CXR. (BTS 1993).

Exclusion Criteria:

1. Pneumonia developed at or after 48 hours, or history of hospital admissions within 1 month of the present admission (to exclude hospital-acquired infections)
2. Patients who are unable to cooperate with the study protocols (such as mentally confused, dementia) or are physically unable to sit or stand independently to carry out the tests optimally.
3. Patients with TOCC associations with infections such Avian Flu or SARS (travel to endemic areas, at-risk occupations, close contacts with index cases, especially with compatible clinical features) or NPA revealed positivity of influenza A/B
4. Patients who are clinically or haemodynamically unstable, such as the need for inotropes, oxygen supplement of >2L/min, any types of shock, etc.
5. Underlying diseases that might affect the coughing effort (such as musculoskeletal diseases, or severe kyphoscoliosis)

60 from each groups. Healthy subjects - recruited from voluntary basis. LRTI - recruited from patient wards with consent. URTI - recruited from out-patient clinic with consent.