Contrail-like formations produced by aircraft over the US, Germany, and Australia, among other places. Certain people believe they are evidence of a vast government conspiracy to expose citizens to bio-warfare agents for some undisclosed purpose.
Willam Thomas, a self-labelled Investigative Reporter who also sells vitamin and mineral supplements to protect against chemical warfare agents, claims they are for weeding out old folk:
"These "weaponized" pathogens target the elderly and other immune-compromised victims whose deaths go largely unremarked. Though I have avoided this conclusion during nine months of intensive research, I now believe that the culling of "unfit, useless eaters" has begun, with an initial goal (based on known mycoplasma and brucellosis mortalities) of 4%-5% fatalities among those infected. While significant over time, such subtle increases in deaths among aged or ailing people already expected to die soon remain statistically stealthy - much less obtrusive than bodies piling up in the streets."
William Bradley, in his book The Gods of Eden, even links chemtrails back to UFO's and the Black Plague (specifically the variant called pneumonic plague):
"Many outbreaks occurred in summer during warm weather in uncrowded regions. Not all outbreaks of bubonic plague were preceded by rodent infestation; in fact, only a minority of cases seemed to be related to an increase in the presence of vermin. The greatest puzzle about the Black Death is how it was able to strike isolated human populations which had no contact with earlier infected areas. The epidemics also tended to end abruptly.
A great many people throughout Europe and other Plague stricken regions of the world were reporting that outbreaks of the Plague were caused by foul-smelling "mists". Those mists frequently appeared after unusually bright lights in the sky. The historian quickly discovers that "mists" were reported far more frequently and in many more locations than were rodent infestations. The Plague years were, in fact, a period of heavy UFO activity."
Chemtrail theory and reportage has seen a recent upsurge in interest lately, after a few years lull. There are many pictures of contrails that may or may not be chemtrails to be found in books and magazines and on the internet. If you have any pictures of contrails, you may want to put them up on a webpage and label them as chemtrails, because you can.
The fear of being poisoned by aircraft may come from the history of actual harm from pesticides by cropdusters in the mid-70s. Run-off from pesticides sprayed over crops also led to many birds of prey, including the Bald Eagle, becoming endangered due to a side-effect causing their eggs to become soft-shelled.
An excellent source for debunking the chemtrail theory, and more information about it, can be found at: http://www.tbtf.com/archive/1999-08-23.html#s06 .