Why is it 'the Corona virus' not 'Corona Virus'?

"Corona Virus" is not the name of the virus which causes the pandemic disease Covid-19, it is the name for the type of virus of which it is an example. This is similar to the way in which "ape" is the type of species to which chimpanzees, gorillas, bonobos and humans belong.

The World Health Organisation has named the specific virus which is causing the pandemic "SARS-Cov-2". If anyone talked about 'the SARS-Cov2' they would be talking about a specific individual virus, perhaps one that they had under an electron microscope at the time.

There many different corona viruses (which are called corona viruses because they have protruding proteins on their outer surfaces and so appear to have a 'corona' round them under an electron microscope) several of which cause the so-called 'common cold'. We refer to SARS-Cov-2 as "the corona virus" because it is the one we are all talking about at the moment, this distinguishes it from all the others.

We don't talk about different species of ape in this way because we can see and recognise individual apes so "the ape" refers to a specific individual (in my case a human, or 'Naked Ape' as zoologist Desmond Morris called us, named Ben).

We can't see individual viruses, at least not without an electron microscope, so we talk about "the corona virus" referring to the whole species called SARS-Cov-2. This is somewhat similar to the way in which Desmond Morris referred to the entire human species as "The Naked Ape" in his book of the same name when it came out in 1969.