Can I say " my computer is infected by a virus ." ?
1. Can I say " my computer is infected by a virus ." ? And What are the other ways to express a similar idea ?

2. "Some websites might carry viruses ."
Is " carry " the right verb to use here ? Or what are the alternatives?

3. " A turtle might retract its head into the shell when you touch it "
Is " retract " the right verb to use here ? Or what are the alternatives ?

Thanks a lot for taking the time to clarify them for me !

Question

Can I say " my computer is infected by a virus ." ?
1. Can I say " my computer is infected by a virus ." ? And What are the other ways to express a similar idea ?

2. "Some websites might carry viruses ."
Is " carry " the right verb to use here ? Or what are the alternatives?

3. " A turtle might retract its head into the shell when you touch it "
Is " retract " the right verb to use here ? Or what are the alternatives ?

Thanks a lot for taking the time to clarify them for me !

Answer

We say 
1) "My computer is infected with a virus.  My computer has a virus.  these are better than by a virus.

2) seems ok but its simpler to just say Some websites might have viruses.  Have/has suggest possession so its a useful verb.

3) is technically correct its what turtles do.  You could say; Turtles might pull their heads in when you touch them.  More natural.

Answer

On number one I would say that is the best term to describe malware on a computer.  I have heard people use malware in different ways, like even "malware infection" but it's not really a popular term.

On number two, again, that is the best term about websites.  You can also use "have".  Not as common though.

On number three 'retract' is the best verb.

Hope I was able to help.