How can a DDoS attack on a single company slow down the entire internet?

The news wires have been reporting the largest DDoS attack in history is taking place today against a company called Spamhaus. I haven’t noticed much of an impact, but the news reports suggest that the effect is being felt by companies including Netflix. How can a single attack have such a wide impact?

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Story added 27. March 2013, content source with full text you can find at link above.