The single most common defense to patent infringement is a counter-attack on the patent itself, i.e., the validity of the patent and the allegedly infringed claims. Even if the patent is valid, the plaintiff must still prove that every element of at least one claim was infringed and that such infringement caused some sort of damage. In case of a medical procedure patent issued after 1996, a U.S. infringer may also raise a statutory safe harbor defense to infringement.
Under certain jurisdictions, there is a particular case of patent infringement, called "contributory infringement" or "indirect infringement". This can occur for instance when a device is claimed in a patent and when a third party supplies a product which can only be reasonably used to make the claimed device.
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