In the stop-and-go rush of modern life, it's easy to overlook the wondrous displays of constancy and rhythm that nature puts before us: Sunrises. The tides. Flu season.

While the last of these examples may seem less than awe-inspiring, the last flu season provided plenty of excitement for investors. Chiron (NASDAQ:CHIR) was supposed to supply the U.S. with about half its flu vaccines last year, but had its manufacturing license pulled, leaving the U.S. high and dry and Chiron's stock in a slump.

For the 2005-2006 season, many of the same players are back. Sanofi-Aventis (NYSE:SNY) is again set to be the largest supplier of U.S. flu vaccine stock. Chiron is angling to get back in the Food and Drug Administration's good graces so it can provide as many as 30 million doses of its Fluvirin vaccine (about 20 million fewer than it was projected to supply last year). Finally, MedImmune (NASDAQ:MEDI) expects to furnish 3 million doses of its FluMist.

But these companies may not dominate the field for long. Chiron's stumble last year evidently attracted new competitors hoping to capitalize on that company's mistake. The most recent example is GlaxoSmithKline (NYSE:GSK). The British firm announced yesterday that it has filed a Biologics License Application (BLA) with the FDA for its Fluavirix vaccine. The vaccine is currently available in 75 countries, and Glaxo hopes the FDA will grant Fluavirix speedy approval so the product will available for the upcoming flu season. Meanwhile, ID Biomedical (NASDAQ:IDBE) is lagging behind, but still intends to be a player. That company is currently conducting clinical trials for its Fluviral product in hopes of submitting a BLA later this year and supplying Fluviral in the 2006-2007 flu season.

Ultimately, multiple suppliers may be good for the U.S. vaccine supply, but not so great for individual companies. The vaccine business is notoriously tricky. Regulatory compliance is a challenge, plants are expensive to maintain, and the single-use nature of the product means that its profits are lower than those of other medicines requiring prolonged and repeated use. Given the nature of the business, respectable returns are predicated on high volumes. If ID Biomedical enters the market next year as planned, all of the flu vaccine market's participants may find the segment too crowded for comfort.

Give this healthy selection of pharmaceutical Foolishness a shot:

Fool contributor Brian Gorman is a freelance writer in Chicago. He does not own shares of any companies mentioned in this article.