The Mirage

Star Sargeant Mann entered the ruined Alien city with his co-pilots Lieutenant Zena and Dr Donna.
Far behind him he had left their crashed  astrocruiser, the Adam - 1 smothered in a black cloud of burning fuel.
"Can you believe this?", asked Mann, looking round him at the ruined alien structures; his eyes illuminated with wonder.
"Believe what?", asked Donna.
"What do you mean what?", replied Mann, "This city. These buildings".
"I don't see any buildings", said Donna, "Do you Lieutenant?".
"I don't see anything but an empty wasteland", said Zena.
Mann started to laugh.
"Will you cut it out", said Mann, looking round at his colleagues, "This is no time to be joking".
"I'm serious, Jack", said Donna, "Theres nothing here".
Mann looked back round him again, closed his eyes and then opened them again but he still saw a ruined ancient city with strange polyhedral buildings of stone and crumbling alien bodied statues.
"So your telling me that none of this is real?", he asked, incredulously, "That you can't see those buildings or those stuatues over there".
Donna, took a small detector device out of a side pocket on her space suit and showed it to Mann.
"Look at the readings, Jack", she said, "This is an empty desert. No buildings and no statues".
Mann took hold of the small device which showed, upon a small hexagonal screen, a glowing, abstract 3-D representation of the area around them and it was true, there were no buildings and no statues.
Mann reached out and touched one of the nearby alien buildings with his hand, brushing dust and desert sand off of its surface and it felt as firm as concrete.
"Then why does it feel so real?", he asked.
"Perhaps you suffered some head trauma when the astroship crashed", said Donna, "Or it could be simple dehydration".
Mann reached into one of the open breast pockets of his suit and took out a sachet of rehydrating fluid, tearing open its foil seal with his teeth before taking a squish of its raspberry flavoured contents.
"Alright", said the Star Sargeant, although still sounding doubtful, "I'll believe you. Maybe I am seeing things".
But then he noticed something else; a moving blur behind one of the tall, hieroglyphic covered pillars to his far left, like a person hiding.
"What about that?", he asked, pointing towards the pillar.
"What?", asked Donna.
"That thing", said Mann, starting to lose his temper, "Theres something alive over there. Theres an alien hiding behind one of those pillars".
Donna started to sound worried.
"I think maybe you need to take some rest, Jack", said Donna, putting a space gloved hand on his shoulder.
But then, suddenly, whatever had been lurking behind the pillar must have fired something at Mann because he was hit in the chest with a hard thud that knocked him to the ground.
"Jack", said Dr Donna, rushing forward and then kneeling by his side, "Are you alright?"
"I've been hit", said Mann.
"Hit?", asked Lieutenant Zena crouching down on his other side.
"Hit!", replied Mann, angrily, "Hit by that thing that you both say doesn't even exist".
"I think he just had a fainting spell", said Donna talking to Zena as if Mann wasn't there. "Must be the dehydration".
"Fainting spell?", said Mann, "What?! Can't you people see I've been shot?".
But then, suddenly, Zena and Donna began to fade; their voices becoming quieter and Mann realized they weren't real.
"I remember now", said Mann to himself, "They both...they both died in the crash".
But then he heard another sound, like a creature with three footfalls walking closer and, dragging himself into a sitting position he saw the alien creature who had been hiding behind the pillar coming towards him.
"Don't be afraid", it said.
"Don't be afraid?", asked Mann, looking the creature over and seeing that it was very humanoid apart from its having three legs, "But you shot me".
"Only with a harmless hypodermic dart", said the Alien, "That released translator Nanobots to your brain and gave you immunization against delusion mites".
"Delusion mites?", said Mann, startled.
"Tiny parasites that live on our planet and cause hallucinations to those they feed on", said the alien helping him to his feet, "They're the reason our great civilisation was reduced to ruins and only a small number survived".
"So there are other aliens...I mean people like you on the planet", asked Mann.
"Yes", said the alien, smiling, "Welcome to our world".