The most visual way to depict the first alien contact is a ship with blinding light descending from the sky or crashing to Earth, followed by sizable beings making an appearance. Usually, these aliens are somewhat comparable in size to humans. The way I see it, the first alien contact is likely to be with much smaller extraterrestrials, whether it's by chance or on purpose.
Space is a harsh place, with radiation, the lack of atmosphere, and solar flares. Anything that journeys a long distance would stand a better chance of surviving if it was small, so it would use fewer resources during the trip. Scientists believe that microorganisms can lay dormant in deserts for hundreds of years before conditions trigger them to awake. Alien bacteria could reach Earth—by accident or on purpose—on a space rock if given enough time.
Last year, there was much scientific controversy over whether bacteria Russians found on the outside of the International Space Station was from space or Earth. Yes, bacteria can survive in space.
Given the rapid advances in nanotechnology, something very small and very smart could also survive.
Bacteria on a meteorite or a piece of space junk could reach Earth by chance. That's a good reason for astronauts to shower well. Tiny visitors could also be sent here intentionally.
There are major challenges with near-light speed travel, and not just in reaching super fast speeds. Imagine what accelerating near the speed of light would do to a large living creature. It would flatten the alien like a pancake. The safest way to accelerate is at the equivalent to the alien's home planet gravity or slightly faster, but that means it would take dozens of years to accelerate near light speed, about almost the same amount of time to slow down. You can't just slam the breaks at 186,000 miles per second.
Bacteria and other microorganisms have more resistance to the "super gravity" environment that extreme acceleration would create. And the ships wouldn't have to be very large. As far as we know, only elements with very little mass-elements travel at light speed, so it would be easier for aliens to produce a microscopic "ambassador" to represent them. They could send out a swarm of them towards every corner of the galaxy, and eventually, they would hit a planet worth inhabiting.
We have sent probes to other planets before people, so why wouldn't they do the same? The longer the distance and the less certainty of the conditions that will be found on the planet, the more it makes sense to send a probe, even a very small one.
Maybe the alien microorganisms would take a liking to our water or our blood and act like a virus. They would need a small foothold, a place where they could survive long enough to serve their purpose.
Think about all the amazing advances humanity has made with nanotechnology, making robots smaller and smarter. Any species that could figure out galactic travel, even for small lifeforms, would probably be highly sophisticated with nanotechnology and probably artificial intelligence, too.
Instead of transporting the full alien species across many light years, it would be easier to do the equivalent of "3D printing": gather the right biomaterial to recreate the alien body, set up a habitat where they can survive, and upload the intelligence. Or bring some alien DNA and clone it.
Could this really happen? Maybe it already has or maybe not. But if you want to know what it would be like to have a hostile alien microorganism living inside your bloodstream, see my novel Silence the Living.