I read once that people who have extremely contagcontagious and deadly illness and need to be operated or sedated often describe the hazmat wearing scientist and doctors. As having huge heads due to their massive protected helmets. And from the position of lights which appear harsher and brighter due to the drugs. Describing the bodies as thin and dark. Which led me to this. Humans hundreds of years in the future have discovered time travel and are going back to better understand their ancient human ancestors. However due to the hundreds of years their germs and viruses are extremely deadly. Although not to the humans of the future. But to the humans of the past with much weaker immune systems to combat these super bugs. Which means if they want to travel to the past with their high-tech equipment and study ancient humans. They have to wear thick strong protective clothing. Such as hazmats. To avoid getting us sick and causing a paradox. The only times they allowed their ships to crash was to test the theory and see if they are infact in the past by crashing shuttles and no doubt leaving some kind of message in side to see if this is infact their own universe. Considering earth relative desolate location in the galaxy. I find time traveling humans far more interesting and likely scenario. Not to say I don't believe in aliens. I just don't think they would care about a planet in the middle of nowhere.