Before the existence of elixirs like Advil, you just had to white-knuckle through work if you were sick. Unless you are bed-ridden, you can find enough drugs at the convenience store to sit at your desk and simulate a fully engaged worker. Advil doesn't stop you from being sick. It just masks ill feelings. Sick people still show up to work and infect their co-workers, who then need Advil themselves, thus feeding the cycle.
The easier we make things for ourselves as individuals, the harder we make things for us as a group. Advil relieves headaches, unless everyone at work takes it, which raises the bar for taking sick days, thus creating another headache.