A man.

In infancy he walks on all fours.

In adulthood he walks on two.

But when he gets old he needs a stick.

But nowadays we have rolling walkers so....

Dan, I've never heard that riddle before, but I would guess you're the letter "e".

a person sick with Ebola

That's "the riddle of the Sphinx."

I have a riddle for you.

I'm...
The beginning of eternity,

The end of time and space,

The start of every end,

And the end of every place.

What am I?

An alien. :)))))