Why was Anne Frank spared?

The controversial journal of revisionist history, the Institute for Historical Review, often draws attention to certain facts about the arrest and detention of Anne Frank to support claims that there was no Nazi program to exterminate the Jews, or at least that said extermination did not always happen in the manner or scale claimed by some people.

One particular fact that interests me is that while the Frank family arrived at Auschwitz-Birkenau in September 1944, young Anne and her sister were never gassed, but rather succumbed to typhus nearly six months later (after being relocated to Bergen-Belsen). I understand it’s generally thought that most Jews arriving at Auschwitz-Birkenau (and many other camps) were gassed upon arrival, particularly if they were unfit for labour. This latter category would almost certainly have included children such as Anne Frank, would it not? If this is true, then why were Anne and her sister not killed immediately upon arrival? IHR’s most prominent detractor, Nizkor, reports that Herman Van Pels, a business associate of Anne’s father, accompanied the family to Birkenau and was gassed upon arrival. Why not Anne too? Why was she allowed to live (for the time, at least), and what did she do during her six months of imprisonment?

I don’t want to turn this into a debate about revisionism or a tirade against IHR; I’m well aware that many of their claims are considered indefensible. I’m just curious on this one point about Anne Frank, as the material Nizkor and other Holocaust sites I’ve read doesn’t seem to have a proper rebuttal for it.

References:
IHR article on Anne Frank
Nizkor article addressing IHR’s claims regarding Anne Frank

I don’t remember if it would answer all of your questions, but Melissa Muller’s book on Anne Frank covers the last months of her life right up to the end.

It’s a bit hard for me to obtain arbitrary English-language books at the moment; can you (or someone else who’s read it) give me a brief summary?

They were teenagers, not children. The Nazis would have considered them old enough to work.

Actually, I just finished the Müller book today so I have it here. Let’s see.

From page 247 of my paperback.

She wasn’t considered a child, and later on it says that she did work too, hauled stones and dug up sod. Most likely they considered her able to work, and through her mother’s help (it also says she passed her bread to her daughters), she managed to stay alive until the typhus struck the camp.

As the Soviet army approached Auschwitz and other camps in Poland the Nazis forced the surviving inmates to march to other camps inside Germany. Many didn’t survive the marches but Anne did make it to Bergen-Belsen. She died of typhus just a few weeks before the camp was liberated.

As the Soviet army approached Auschwitz and the other camps in Poland the Nazis forced the surviving inmates to march to other camps inside Germany. Many died en route but Anne and her sister Margot survived the march. Anne died of typhus at the Bergen-Belsen camp in March just a few weeks before it was liberated.

Oh, right. Duh. I only ended up scanning that chapter so I guess I should’ve read a little closer to see that they went to the Bergen-Belsen camp.

Anne and Margot could not withstand the typhus, given the cold, lack of decent clothing, poor sanitation, malnutrition, lice, rats–all of which contributed to the decline in their immunity.