The Apollo 11 astronauts quarantined after returning to Earth. Here's why
The reason for that caution was twofold. The astronauts may have been exposed to harmful bacteria or other unknown elements. The mission was the first time humans had interacted with the surface of another celestial body. Doctors closely monitored the astronauts, while a separate team tested and studied the lunar rocks and dust brought back by the astronauts. There was also a desire to protect any potential life that may have been brought back from the moon in the lunar samples.
Source: cnn.com