I learned a reason why vaccines may contain mercury from one of my biochem profs. Mercury might be the only catalyst that exists for the only reaction that leads to the product. Having to purify the product of mercury to ppb concentrations is extremely difficult, and is also one of the reasons your prescription drugs are so expensive. So Big Pharm isn't completely evil XD
I just learned on wiki that mercury is added to vaccines as an anti-bacterial. So scratch out what I wrote about mercury's purpose in vaccines. Although I will leave it in because I don't think it's something that people realize - and chemists are trying very hard to develop new ways of conducting the same reaction without toxic reactants like mercury - it's called Green Chemistry, look it up all you science nerds as it's pretty cool and related to sustainability and environmental protection.
I just learned in biochem that toxins that kill bacteria by shutting down their ability to create proteins. They target bacteria because bacteria and humans have different machinery and the toxin only works on bacteria. And I just realized why that would be a bad idea - since bacteria can always develop resistance to antibiotics. But apparently mercury works in much the same way, preventing bacteria from reproducing and creating proteins. So I am thoroughly confused as to why mercury is used instead of an organic antibiotic.
But my conclusion is this: scientists should continue working on developing an antibiotic potent but safe for human use. I don't know how possible it would be, especially since antibiotics block fundamental processes for life.