Thanks for a very good explanation of the Shan Han Lun. Another translation for this is Cold-Induced Evils. This was an early theory in Chinese medicine for dealing with colds and flu-like Evils. Any of you who have decocted herbal teas for yourselves have learned that decocting a tea or preparing other herbal rememdies can be time consuming. For one thing, you have to let the herbs soak in water and then cook them.
Some herbal teas require hours of cooking time.
What is appropriate for one stage of a cold or the flu will make a person sicker during another stage. For example, in TaiYang Chill, you want to promote sweating, but this is a no-no in TaiYang Wind and in other stages. A lot of bitter, cold herbs typically are used to treat the Yang stages which are characterized by fever, but are contraindicated for the Yin stages. By the time an herbalist gets the herbs prepared for one stage, the client may have passed to another stage in which the herbs are inapproriate. It's a part of being able to effective treat in the present.