In 1982 a law was passed by the U.S. Congress which included a section which made (or could be interpreted to make) contact with ETs illegal. More exactly, the law authorized the detention and quarantine of any person who has been "extra-terrestrially exposed", which is defined as a condition of having either,
Quarantine could be as long as NASA wished it to be, and was to be enforced by armed guards. The decision as to whether or not a person was "extra-terrestrially exposed" rested with the NASA Administrator (or his agents) and could me made without the need to observe the usual niceties of the legal process, such as arrest and trial.
This concern was raised in UFO circles, and finally someone looked into the matter. It seems that the law was repealed by NASA in 1991. The full story, including the original message of R. Solomon, which quotes the entire law, is at Federal ET Law. NASA's official reason for repealing the law is that it,
Yet NASA still has manned missions to other planets (in particular, Mars) planned, so what has changed? And was this law originally drafted (ambiguously) to keep us safe from alleged ET viruses or, as R. Solomon suggested,
Fortunately this law is
now off the books, and it seems the government is adopting a
different strategy: using Hollywood to prepare the population of an
invasion of Earth by ETs (with the U.S. military of course coming to
the rescue, at the small price of imposing permanent martial law
upon the country).