"up to now" and "up till now"

> up till now一般是和现在完成时搭配的，表示个时间一直持续到现在。
> Until now, doctors have been able to do very little to treat this disease.

"up to now" is different from "up till [or until] now". Whenever you have the word "till" or "until", it implies that something did not happen before that point in time BUT IT DID HAPPEN AT THAT TIME! The above sentence you gave means that the doctors now ARE ABLE TO do something significant to treat this disease. "up to now" may or may not have that implication or side effect. It's much less used. In fact, I always say "until [till] now" to HAVE that side effect and "so far" to AVOID that.

Answers.com and thefreedictionary.com claim that "up to now" can only be used in negative sentences (see e.g. www.thefreedictionary.com/up+to+now). That's not right. You can see people's usage by searching on Google for this phrase (wrap the three words in double quotes), and ignore the cases where the phrase "be up to" is meant.