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The coronavirus is now turning legal -- a quarantined cruise in California has been sued for allegedly putting passengers at risk ... when they knew the virus was running rampant on the ship.

Ronald and Eva Weissberger -- two elderly passengers who are currently on the Grand Princess cruise ship that was quarantined and stranded out in the Bay Area waters for at least a week -- just filed suit against the parent company, Princess Cruise Lines.

In the docs, obtained by TMZ, the couple claims the PCL was grossly negligent in allowing them and thousands of other passengers onboard in late February ... when the ship's staff knew damn well at least 2 other passengers with coronavirus had just disembarked from a previous voyage -- and at least one of them died as a result.

They go on to allege that PCL had an awful screening process -- claiming that the new group of passengers were only asked to fill out a basic piece of paper stating if they were sick or not. There was no other testing, so claim the Weissbergers.

Adding insult to injury, the couple goes on to claim that at least 62 passengers onboard the current cruise were also on the prior infected voyage. BTW, the Grand Princess was finally allowed to dock Monday in Oakland.

They have a damning quote that reads, "Incredibly, not one of those sixty two passengers or crew members who were mixing and mingling with the infected prior passengers were ever examined during the instant voyage until being tested for the virus on Thursday March 5, 2020, two weeks after the ship sailed."

Bottom line ... the Weissbergers say they would have NEVER stepped foot on the ship had they known all these potentially infected people were all around them. Now they're traumatized from the fear of developing coronavirus ... especially because they're older and already have underlying medical conditions. They claim PCL put profits over the safety of their passengers.