(NEXSTAR) – On Wednesday, the Centers for Disease Control and Prevention said 88 cruise ships were either under investigation or observation due to COVID-19. Four other vessels were also being monitored by the CDC.

Concerns over outbreaks of COVID-19 on these ships have risen recently, just six months after cruise lines were given the green light to begin sailing again. None of the ships so far appear to have so many cases that they would overwhelm medical resources on board and require a return to port. However, some have been denied entry at foreign ports.

Ashley Peterson, a Carnival cruise passenger, made an appearance on “Morning in America” to talk about her experience aboard a ship with an outbreak. The Asheville, North Carolina resident’s Christmas cruise departed from a port in Miami for an eight-day trip around the Caribbean.

Soon after setting sail, the ship was denied entry to some ports because of passengers who tested positive for COVID-19.

“I learned about the outbreak before they announced it,” Peterson said. “When I asked, they wouldn’t confirm that there was COVID on the ship. And they told us the next day when we were denied entry.”

Peterson said the conditions on the ship were a virtual petri dish with little to no COVID precautions.

In a statement, Carnival Cruise Line said, “Carnival Freedom followed all protocols. After a small number on board were isolated due to a positive COVID test. Our protocols anticipate this possibility and we implement and adapt them as necessary to protect the health and safety of our guests and crew.”

According to the CDC’s ship tracker, the Cruise Ship Color Status, Carnival Freedom is one of the dozens of vessels marked with a yellow status, meaning cases of COVID reported on the ship have met the threshold for an investigation by the CDC.

The color-coding system, displayed using a chart updated several times a week on the CDC’s website, is broken down into five classifications: green, orange, yellow, red, and gray. To determine a ship’s status, the CDC relies on surveillance data from the previous seven days and findings from CDC investigations.

Here’s what each color means:

As of Wednesday night, there were 19 ships in the green category, four in orange, and 88 in yellow. There were no ships in the red or gray status.

The CDC said cruise ships “will always pose some risk of COVID-19 transmission” and recommends all travelers, whether passengers or crew, get fully vaccinated against the virus. Those that are not fully vaccinated are encouraged to avoid traveling on any cruise ships, either nationally or internationally.

Various cruise lines, including Carnival, Disney, Norwegian, and Royal Caribbean, have ships on the CDC’s Cruise Ship Color Status., which you can find here.

The Associated Press contributed to this report.