excerpted from:

http://www.scientology-lies.com/faq/teachings/aliens.html

Do Scientologists believe in space aliens?

Scientology does teach that many of our problems are caused
by the spirits of space aliens that are stuck to our bodies.
However, that teaching is considered confidential and is
only taught to Scientologists who have reached a certain
level, known in Scientology as OT III - so a lot of
Scientologists are actually unaware that this is an
important Scientology teaching.

Scientology Teachings about Space Aliens What, exactly, does
Scientology say about space aliens?

In the materials for OT III (Operating Thetan level 3), L.
Ron Hubbard writes that, 75 million years ago, the head of
the Galactic Federation, made up of 76 planets, was a being
named Xenu. Faced with an overpopulation problem, he brought
beings to this planet, blew them up with hydrogen bombs, and
packaged them. Their spirits now infest our bodies: he says
"One's body is a mass of individual thetans stuck to oneself
or to the body." Scientologists at this level try to rid
themselves of these thetans (spirits) by helping each one to
remember the painful experiences of being blown up like
that.

There are other Scientology teachings that relate to space
aliens. The book "Have You Lived Before This Life?",
described in Scientology advertisements as "a cold-blooded
account of your last years", contains dozens of case
histories of memories experienced by Scientologists, some of
which include adventures in outer space.

Even the glossary in Scientology's What Is Scientology?
contains a definition for the phrase "space opera", which,
according to Scientology, relates to periods on "the whole
track" (that is, our whole history, going back through many
lives and millions of years), and it contains "space travel,
spaceships, spacemen" and so on.

I am an ex-Scientologist,  left Scientology in 1975,  long before I found
out about the space alien aspect.

However,  with respect to viral diseases,   my understanding is that they
will go along with the medical establishment.    It is in the area of mental
illness they take unorthodox views.

Patrick