Yes, our DNA could be altered to deal with viruses, but how many millions of different types of viruses and bacteria are there just on Earth? Look at the influenza virus — every type of influenza mutates every single year to the point where we have to have a completely different vaccine every year. This doesn’t mean it’s impossible that we might be able to land someplace and find our bodies are unaffected and unthreatened by the local pathogens, but…evolution, y’know? Evolution is likely a universal factor.