(NewsNation Now) — Excursions on at least four cruise ships based out of Florida were canceled or turned away from ports this past week due to COVID-19 outbreaks.

A Carnival cruise ship from Florida was denied entry into Bonaire and Aruba after a few passengers tested positive on the ship. Instead of making three stops, the ship made only one. 

The U.S. Centers for Disease Control and Prevention said Tuesday is investigating or observing 86 cruise ships with reported COVID-19 cases on board, according to its website,

So why are cruise ships more vulnerable to outbreaks of diseases like COVID-19?

Here are the facts:

• On a cruise ship, passengers are in close proximity to each other and share the same air. For most cruise lines, the air is not filtered and purified like it is in an airplane so diseases linger longer.
• Some cruise liners have added air purification systems since the spread of COVID-19, but they’re still below the standards of airplanes.
• You’re more likely to be exposed to contaminated food if there is any onboard since the food is prepared by a single kitchen.
• The average age of a cruise passenger is older, which means collectively more people are vulnerable to disease.
• Once a disease outbreak does occur, it can be very difficult to eradicate.

It’s important to note that millions of people sail on cruises every year without catching a disease.

Watch the full Smart Board report in the video player at the top of the page.