Thanks everyone for their patience waiting for part 4. My new job has me working 55 hours most weeks now, leaving a lot less time for writing.
I've tried to give a quick re-cap at the start, but I'd advise reading parts 1-3 first.
Usual disclaimer - all characters are adults over 18.
Enjoy :)
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It had been over a year since 'patient zero'.
A month since the world learned the truth, and society was changed forever.
A virus had emerged - transmitted sexually to women, and airborne via coughing to men. By the time the governments of the world officially admitted the virus's existence, over 95% of adult males, and 65% of women were believe to be infected.
For men, there were no symptoms, other than a slightly increased libido, and being more open to persuasion.
However, for women, the virus was very serious. Within a two weeks of infection there would be extreme dizziness, the coughing of blood, then moving onto loss of consciousness, a coma and death.
Cure - none.
There was a treatment to stall the virus, but it wasn't a cure. Men's semen. The same fluid that caused the infection contained an antivirus that halted the virus, and even scaled back some of the worse symptoms. And there was only one way to administer it - vaginally. Nothing else worked. Once infected, regular, unprotected sex was the only answer.
Exactly one month ago, 18 year old Emma had admitted to her 20 year old brother Simon that she was infected. Reluctantly at first, he'd fucked her to provide the antibodies needed to save her life. Since then they'd tried to fuck at least once a day. At first, that had simply been to heal her, as the virus had almost put her in a coma. Now, it was more because...well...they loved fucking each other.
For Emma, it had always been her fantasy, ever since being old enough to fantasize sexually. It was her brother, or her father, fucking her until her pussy was red and full of cum. In some ways this virus had been a gift from above, allowing her to fulfil her deepest desires.
For Simon, he'd never thought of his sister sexually before the virus. He wasn't unattractive, and had rarely struggled to find a pretty girl to share his bed at university. But the more taboo sex with his sister he got, the more he enjoyed it. Since the infection, she was the only girl he slept with.
It wasn't easy for them. They still lived with their parents. (Simon's university had yet to reopen after the viral outbreak. Instead he was attending online lectures and sending assignments in via email.) Their father Peter was a 45 year old hedge fund analyst. Their mother Yulia was a semi-professional tennis player, originally from Russia. Simon and Emma constantly looked for moments when they would both be out, even for just ten minutes, or wait till the house was quiet in the morning or the late evening. Both brother and sister were sure their parents suspected their secret sex, and it had become difficult for them to be alone.
Often, Emma would hear Yulia and Peter fucking through her bedroom wall, and she'd quickly call her brother in, knowing they had at least half an hour undisturbed. She loved to have her brother's cock inside her, while hearing her parents fucking through the muffled bedroom wall. Two weeks ago had been their parents' wedding anniversary, and it wasn't until about 3am before her mother's moans of pleasure finally died away. That was the first time she'd been able to not only get a creampie from her brother, but after he'd recharged, give him a blow-job and finally taste the cum he'd been planting in her daily.
She wasn't worried about pregnancy - her older sister had given her some birth control pills. Anastasia now came round to the house once a week. She knew exactly what her two younger siblings did - in fact she helped set it up. And when Anastasia came to visit, they always found an excuse for the three of them to be alone.
Normally, the sibling sex was a quick, rushed affair, worrying about being caught at any moment. But when the three of them were together, Simon and Emma really had time to enjoy each other's bodies. Simon always went down on his little sister's tight pussy, enjoying her taste as the wet juices flowed. To her surprise, she'd learned her brother was into feet, and she often let him suck on her toes at some point in the love-making.
Anastasia never joined in - she just watched, openly playing with herself while gazing at her younger brother thrusting his cock into her younger sister. Sometimes she'd instruct them to try new positions, or made sure they were keeping the noise down to avoid discovery. Then afterwards, she would go back home to her boyfriend.
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Having sexual freedom whenever their big sister came round had given Simon and Emma a good idea. They had started inviting friends round for overnight sleepovers. Their parents always relaxed their guard when guests were around. One sibling would go to bed early, while the other would watch movies in the living room with their guests (after the parents had gone to bed), and at some point the 'host' would sneak out for one reason or another. Emma would leave the movie because she hated one scene in the film. Simon would excuse himself because he had an upset stomach. Then they had 10/15 minutes without interruption.
This evening, Emma had invited Karen round from her church youth group. They had decided she was perfect - she wasn't the sharpest tool in the shed, and would probably not notice if Emma disappeared for even 30 minutes. The only downside was that she was also the twin (non-identical) of Callum, Emma's ex.
Things were going well. Their parents had already said goodnight to the two girls in the living room. Simon was in the kitchen, making a snack before he 'went to bed'.
Karen was talking over the movie (some awful rom-com) when she coughed into her hands. She tried to wipe it away, but Emma saw it. It was full of blood. Karen's black skinned face went red.
"Karen?" asked Emma in surprise. "Are you... infected?"
Karen looked down at the carpet, her face welling up in unplanned tears. "Yes...no...I don't know."
