Generally speaking, the large trails are normal jet aircraft water condensation trails (contrails). Some of the smaller streaks in the sky could be caused by small aircraft seeding clouds to produce rain. In the US, local state governments usually regulate the cloud-seeding industry.

Cloud seeding supplies here:

http://www.nawcinc.com/photos.html

I think this panic among some people started back in the ‘70s or ‘80s when a newspaper broke the story about the US military spraying San Francisco with some minor bacteria during a cold-war chemical warfare test of the 1950s or '60s. Supposedly, a few people in San Francisco got sick from the bacteria and a couple of them died.