What would a "miracle cure" drug look like?

I'm a molecular biology student in college right now and have been learning about the specific molecular pathways by which common drugs kill/incapacitate pathogens. Would it be possible to create some sort of Franken-drug with an antiviral domain, antibiotic domain, antifungal etc., all stuck together in a single molecule? That way you wouldn't need to be tested for a specific disease, you just take the pill as soon as symptoms appear. What would be the downsides of this?