There is no grammatical difference between two nouns. If they have different meaning, then there is a lexical difference.
Viruses are (debatably) not alive while bacteria are; meaning they are smaller and require a living host to survive. Bacteria are killed with antibiotics, but viruses are not.
were killed suggests that they were killed and that's the end of it.have been killed suggests that more killing is going to occur in the future
There is no difference in the meaning. Lettonia is Italian for Latvia.
there the same meaning
No
according to me ,there is no difference between yet and nevertheless since both have same meaning .
meaning of meal
both have same meaning.
what is the meaning of sum? and total
None, same meaning.
There is very little difference unless you take away the 'for' :)