The kanji 「疾」 originally meant "illness" or "disease" in classical Chinese.
That meaning still appears in words like:
「疾病」(shippē) = disease
「持病」 uses a different character, but similar semantic field
「痼疾」(koshitsu) = chronic illness
However, 「疾」 also developed another meaning: fast, rapid, swift.
This happened because in ancient Chinese the character was associated with things that move or progress quickly, including:
rapidly spreading illness
swift movement
fast arrows or wind
So in 「疾風」(shippū):
「疾」 = swift / rapid
「風」 = wind
Together it means "a swift wind" or "gale."
The same "fast" meaning appears in:
「疾走」 = sprinting, running fast
「迅疾」 = rapid, swift
So the character has two related historical meanings:
illness / disease
swift / rapid
Many Chinese characters and Japanese kanji carry multiple meanings that evolved from older usages over time.