The strange visitor

Everything had already happened, everything had finished. Although she had been told that would happen, she was terrified. She had also been told that she was immune but she didn't know what she was immune to. 

The whole world was quiet. Just the sound of breath could be heard. After apocalypse everything and everyone had died, however, there she was, immersed in solitude.

Then, someone knocked the door, startled and relieved for a visitor she rushed downstairs and opened it. It was a middle-aged man. - "You're coming with me" he said. He dragged her into a car. - "I want your DNA and some people your blood". Now, she was more frightened.