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Alien life would almost certainly be alien to the point that viruses couldn't infect them even if they did share some chemical processes. We share some of the same chemical processes with plants, but no plant virus is ever going to infect a human. A gulf exists between plants and humans, even though both are distantly related. Even if an alien looked like us, they would be quite different.
The panspermia hypothesis is plausible, but even if it turns out to be true, life on the two planets would have a common ancestor as distant as bacteria. This might lead to greater similarity between life on the two planets than if life had begun separately, but they would still be quite alien.
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