As states reopen and countries move to relax some border restrictions and quarantines, get ready for a more complicated airline check-in procedure.

United became the first U.S. airline to require all passengers to self-certify their medical conditions before they’re allowed to board. You have to state that you’ve not experienced Covid-19 symptoms, including high temperature, coughing, shortness of breath, chills or muscle pain, 14 days prior to flying. You also have to certify that you have not been denied boarding by another airline.

Other airlines are expected to follow suit.

Why is United doing this? One argument is that it’s worried about its own liability and protecting itself against future lawsuits claiming that the airline boarded passengers without knowing — or asking — their medical conditions.