As you may have heard, Craig Biggio is in the midst of an 0 for 20 batting slump stretching back to September 3rd. His last hit came in his final at-bat on September 2nd against the Mets. This is the 5th time in his career he's had 20 or more consecutive at-bats without a hit, and it will come as no surprise to readers of this sight that during the spans of those 5 hitless streaks, he was only hit by a pitch once. Biggio went 0 for 20 from the 1st inning of the game on May 2, 2002 to the 9th inning on May 9, 2002 when he singled against the Phillies. He was plunked during that streak on May 4th by Jeff D'Amico.
Clearly, getting plunked prevents hitless streaks from going longer than 20 games. Biggio's longest hitless streak was an 0-27 slump from September 19, 2002 to the 8th inning on September 28, 2002. He's had two 0-23 hitless streaks, but the first one spanned an offseason - from September 25, 1998 to April 10, 1999. Spanning two season makes streaks seem - less streaky? The other 0-23 stretch ran from the 5th inning on September 19, 2004 to the 2nd inning on September 27, 2004. You may have noticed by now that 4 of Biggio's 5 hitless streaks of 20 or more ABs have occured in September, and those are the same 4 (including the current one) in which he did not get hit by a pitch - by no coincidence, September is also the month in which Biggio has his worst career batting average (.255), and the fewest plunks of any full month of play (34).
Clearly the HBP is the key to Biggio's offense, and opposing pitchers have been keying on this and refusing to throw him anything he can get hit by. As a result, the hitless streak follows the get-hitless streak.