Cruise ships can prepare to set sail again beginning Sunday under a conditional order issued by U.S. health officials that aims to mitigate the risk of Covid-19 transmission at sea by requiring a host of measures, including testing and quarantine, all designed to keep crews and passengers safe.No ship will set sail with passengers immediately, and the cruise tourism industry may not rebound anytime soon. Under new guidelines issued by the Centers for Disease Control and Prevention on Friday, companies must be certified to sail by proving they can operate safely with crews onboard. To do so, they must carry out a simulated journey, or a number of simulated journeys, with unpaid guest volunteers or crew members playing the role of passengers.The simulated journeys must provide regular onboard activities such as meal service and entertainment in common areas of the ship, while providing enough space for social distancing. Ships will be required to have laboratory capacity to ensure that routine testing for the coronavirus can be carried out at regular intervals, as well as when anyone embarked or disembarked from the vessel. Both crew members and passengers will wear masks in public spaces.Symptomatic travelers on the ship will have to be isolated, and remaining passengers quarantined, and the efforts will be evaluated by the agency in order for operators to obtain certification to sail with commercial passengers.The C.D.C. outlined the phased approach, acknowledging in a statement: “Cruising safely and responsibly during a global pandemic is very challenging.”The federal health agency had tried to extend until next February the no-sail order it had issued last March. But the White House blocked the order in an apparent attempt to avoid alienating the powerful tourism industry in Florida, one of the swing states that could determine the outcome of the presidential election on Tuesday.“This framework provides a pathway to resume safe and responsible sailing,” said Dr. Robert R. Redfield, the director of the C.D.C. “It will mitigate the risk of Covid-19 outbreaks on ships and prevent passengers and crew from seeding outbreaks at ports and in the communities where they live.”The framework outlined by the C.D.C. built on a report issued on Sept. 21 by the Healthy Sail Panel, an alliance of industry leaders and nongovernmental experts convened by the Royal Caribbean Group and Norwegian Cruise Line Holdings, which met over the course of several months, ultimately developing 74 recommendations. C.D.C. representatives acted as observers at the meetings, Dr. Martin Cetron, director of global migration and quarantine at the C.D.C., said.The agency later used the framework to develop the guidelines for conditional sailing that include the phased approach to resuming operations, Dr. Cetron said.Observers will monitor and evaluate the mock journeys to ensure adherence, he added. “If the outcome is not as desired, one has to ask: Is the plan not good enough, or is implementation not good enough?” Dr. Cetron said. “This is a virus that can be very unforgiving of a mistake.”“We all recognize this virus is a formidable foe, and we’re going to be living with it for a while, and we need to adapt our systems to have maximum impact,” he added.Ships will have fewer guests than in the past, and both crew members and passengers would be required to wear masks and to maintain social distancing, Dr. Cetron said. At first, new crew members joining a ship would not only be tested before boarding, but also be quarantined for 14 days. The crew would also be quarantined for 14 days before disembarking.The quarantines would not apply to passengers, however. The C.D.C. said passengers would instead be tested twice before boarding, he said. The guidelines will continue to be improved and “tweaked” along the way, he added.The world’s major cruise lines have been idled for months under no-sail orders as the pandemic swept around the world, after tourists and crews aboard ships like the Diamond Princess docked and were stranded for weeks as infection rates soared onboard.Many cruise lines, like Royal Caribbean, had already announced they would not resume sailing until at least December. Some have canceled future sailings — Carnival Cruise, for example, has canceled all sailings through Dec. 31, as well as some sailings in 2021 and 2022. But with cases rising to record levels in the United States, and European countries initiating new lockdowns with surges of infections spreading, an imminent return to cruise-ship travel remains in doubt.The no-sail order had been extended several times since March but is set to expire on Saturday.The C.D.C.’s website says that scientific evidence suggests cruise ships — which bring travelers from around the world together to live in close quarters with crew members, where social distancing is hard to maintain — “pose a greater risk of Covid 19 transmission than other settings,” and that outbreaks onboard cruise ships “pose a risk for rapid spread of disease beyond the voyage and into communities across the globe.”Asymptomatic and mild illnesses can easily go undetected, allowing for covert spread of the coronavirus, which spreads through microscopic droplets in the air. Even if cases are identified, isolation and quarantine are difficult.One notorious example was the outbreak that occurred on the Diamond Princess after it took on a single infected passenger on Jan. 20. A month later, more than 700 of the 3,711 onboard tested positive. Thirty-seven people required intensive care, and nine died.