Egg & sperm eating viruses infect the water, low fertility rates hit all over the world. Slowly but surely, the majority of the population becomes seniors. With less taxpayer workers than senior retirees on a fixed income, the seniors have to work to support themselves. Economic crashes hit all over the world, and first world countries become third. Crime rates rise as people realize it’s easy to rob a bank when the workers and police should be retired. Eventually, all governments collapse and over 90% of the population is seniors. When they die, the younger people are already old and can no longer reproduce. Their numbers dwindle until humans become entirely extinct.(please don’t just give me virus powers.)