No, disease names are not proper nouns, although diseases named after people keep the capitalization of the person's name (Münchausen syndrome). The scientific (Latin) names of disease-causing organisms follow the standard rule of Genus species.
If you've seen "Celiac Disease", it's just because of an unfortunate tendency some People have to capitalize Random Nouns as if English were half German.
Don't think of Australian dollar as a single proper noun. Instead, think of it as a combination of two words, meaning "a dollar that happens to be Australian":
Australianproper adjective + dollarcommon noun
There are lots of kinds of dollars, and we use adjectives like Australian and Canadian to say which sort we're talking about more specifically. There are lots of kinds of rupees, too, and we use adjectives like Indian or Indonesian to specify which sort we mean. Something similar is true of the Japanese yen and Chinese yuan (which are, by the way, etymologically the same word).
Note that in the preceding paragraph I talked about kinds of dollars and kinds of rupees. I didn't just say dollars and rupees, because an individual dollar is a piece of currency! If I have five dollars, I have either five one-dollar bills or I have some other denominations that happen to add up to five dollars. There is no singular dollar—there are many dollars!
Even if the US dollar were the only dollar in existence, the word dollar still wouldn't uniquely identify a single entity. And the same is true of the word cedi. If I have two cedis, I have two hundred pesewas. There is no singular, unique cedi to which the word refers. So semantically speaking, we have no motivation to treat the word as a proper noun. And syntactically, we don't: I can say a cedi or three cedis, just as I can say a dollar or three Australian dollars.
Ultimately, even if it made sense to treat cedi as a proper noun, the fact is, we don't use it like one, so it isn't one. Any additional explanation is just icing on the cake.
Best Answer
The convention in English (and it may be different in other languages that use the Latin script) is that names of animals, plants and (by extension) viruses are not capitalised, unless part of the name comes from a proper noun.
So we have "blue tit" or "dog rose", but "Steller's sea eagle" and "African elephant" because Africa and Steller are the names of a continent and a person respectively. Ebola is capitalised as it is the name of a river in Africa (the Ebola river is a tributary of the Congo).
We also (again by convention) capitalise the scientific names of above the level of genus. So the blue tit is Cyanistes caeruleus in the family Paridae (the tit family) and the class Aves (birds).
Now "coronavirus" is not the scientific name (that is the subfamily Orthocoronavirinae). The particular one doing the rounds now is a Betacoronavirus. Instead "coronavirus" is the English name and so follows normal capitalisation for animals, plants and (by extension) viruses.
Names of viruses are often formed as single words with the suffix -virus. This is perhaps influenced by scientific use. There are rhinoviruses, noroviruses, adenovirus and so on. Again there isn't a logical rule, that is just how it is done. The virus that causes Ebola is known as ebolavirus (and note the downcasing when Ebola becomes a prefix).