Patents can be obtained in most foreign countries. Infringement occurs when one practices the invention described in the claims. Knowledge, or lack thereof, is generally irrelevant to issues of liability. Independent development is not a defense. Knowledge of the patent by the infringer, however, can be relevant to damages.
Many companies do new product clearance searches to avoid law suits.
There is a requirement that the invention be completely disclosed. Failure to disclose will invalidate the resulting patent. One cannot maintain information important to the patent as trade secret if the information was known as of the filing date. Anyone is free, however, to engage in such activities unless there is a law prohibiting it. The prohibitory laws of greatest concern include FDA regulations, firearm and explosives regulations, and patent laws whereby one's invention improves on another's patented invention.