I never knew that about HIV/Aids and lycanthrophy, though looking back on the books and the films, it makes sense it's a metaphor for Lupin's illness. What an amazing writer this woman is!
As for the Boarhound; it's not necessarily just a Great Dane. It is typically a large breed of dog, such as a Great Dane, used to hunt large game. Dogs used to hunt wild boar (sometimes they have been used this way for centuries); as mentioned the Great Dane, the Boxer, the Fila Brasilero, The Dogo Argentino, the Russian Laika dogs, the Akita, the Cane Corso, Bulldogs, Pitbulls, Staffordshire terriers and the breed used in the films; the Neapolitan Mastiff. These large, heavy dogs were known as "catch dogs" while smaller and lighter dogs, such as the Rhodesian Ridgeback, several breeds of Curs, the Catahoula and the Plott Hound were also use in boar hunting and were called "bay dogs". All dogs used in hunting wild boar, of any breed or mix, were known as Boar Hounds.
I mean, i guess it would get worse, because lupin gets increasingly worse throughout the books. Not so much the films, but he gets greyer and shabbier in the books. So one can only assume that it does get worse. Just how and why i guess we are just left to speculate.
On the HIV/AIDS thing,I think she compares it to lycantrophy because it works in a similar way.People with HIV/AIDS are often shunted by other people,and find it hard to blend in with others,much like a werewolf.It is a disease they can't get rid off,and soon becomes a part of them.
And as for a boar hound well,=imvula already explained that.