Proprietary software on Top of free software

Dear penguins,
I have a curiosity:

how is it possible for a company to develop a Closed Software on top of 
an Open source Platform such as Linux.
Most of the licenses used are viral (GPL), in term that if you use code 
(library also?) open source in your Software this should be Open source 
too. It seems LGPL is an exception.

There are already many commercial (and not open source) applications 
for Linux (Oracle and so on) but in which  way  this software is not 
"infected" by the licensing terms of the Platform where it is developed 
and run?

Thanks, Grazie

Matteo


P.S.

the terms  "Viral" and "infected" are used in this email  absolutely  
in their positive meaning!

P.P.S

don't ask me what is the positive side of a Virus! 

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