If you know of anyone who has dengue fever, ask him/her to drink lotsa fresh and unchilled watermelon juice. It is a natural remedy which is effective. This advice was given by a trusted TCM practitioner my family goes to – 王如田医师 @ Yio Chu Kang road. Just a bit of background information about him.
My paternal grandfather was diagnosed with kidney failure in the early 1990s. Doctor said that he had to undergo dialysis to survive. He refused and insisted that he would rather die than to undergo dialysis. Somebody then recommended 王如田医师 to him. 王医师 gave him a prescription (I remember one of the ingredients was scorpion!), supposedly to 以毒攻毒 （use poison to counteract poison). He survived on this prescription without undergoing dialysis for the next 10 years. He passed away in 2005 as he stopped taking that prescription after his condition stabilised for so long…
Anyway, the above is just to illustrate the credibility of this TCM doctor. 王医师 said that he had treated a lot of dengue cases with success. A simple remedy is to drink lotsa watermelon juice. I also found similar evidences on several websites and forums:
Some of the websites also said that papaya juice and pegaga juice are equally, if not more effective!
Well, Vit C sure plays a role to boost one’s immune system. But I think there could be some other stuff present in these fruits that make them a natural cure for dengue.
[Note: The above is not meant to replace a medical doctor's advice.]