Flu adjectives are listed in this post. Each word below can often be found in front of the noun flu in the same sentence. This reference page can help answer the question what are some adjectives commonly used for describing FLU.
annual, asian, avian
bad, bird, intestinal
new, pandemic, seasonal
spanish, stomach, swine
Hope this word list had the adjective used with flu you were looking for. Additional describing words / adjectives that describe / adjectives of various nouns can be found in the other pages on this website.