<mode=smartass>
Latin is not my native language <g> but I have been told that virii is wrong for at least two reasons:

1. Plurals of words ending with '-us' is made by turning the 'us' into 'i'. Octopus -> octopi. Virus -> viri. You would need a virius to turn it into virii.
2. The word 'virus' means 'poison' in Latin, which is an uncountable noun. It does not even have a Latin plural form.

The modern use of the word 'virus' does have a plural but it is no longer a Latin word and should follow the rules of grammar of whatever language it was imported to. 'Viruses' is fine, as is 'virukset'
</mode>

Otherwise you are absolutely spot on. Viruses are specialized pieces of genetic code that work only when attached to another piece of specific genetic code. They evolved hand in hand with their hosts. Even on earth, inter-species contamination is rare. Conversely, even within a given population of any given species you get individuals with a slightly different genetic makeup that makes them immune to that particular strain of virus.

An alien virus is not impossible but very, very unlikely.

On the other hand, it does not mean that there is no danger from space. Cyanide is not 'alive' by any definition but a few milligrams can kill you in seconds. There may be literally millions of yet unknown chemicals with even more lethal capabilities. A piece of rock from the sky carrying one of those could well be the curtains.