Device and method for non-invasively monitoring asthma and other respiratory diseases, as well as non-respiratory diseases. The method includes collecting condensate from a subject's breath, testing the condensate to determine its acetic acid/acetate level or concentration, and evaluating these properties to determine the presence, absence or status of a respiratory or non-respiratory disease in the subject. The method may also include, prior to the testing step, standardizing the volatile substances that may be present within the condensate in a degassing or gas standardizing step. The device includes a mouthpiece apparatus configured to receive breath from a subject, a condensation apparatus to condense the subject's breath and produce a condensate, and a collection apparatus having a collection chamber containing means for testing the condensate to determine the acetic acid and/or acetate concentration. The device may also include a system for removing or standardizing the volatile substances that may be present with the condensate. The device and method may be utilized without condensing a subject's breath and collecting the same.