WASHINGTON — Forget mosquito bites. Volunteers let researchers inject them with the dengue virus in the name of science — and an experimental vaccine protected them. Next up, scientists plan to use this same strategy against dengue’s cousin, the Zika virus. It’s called a human challenge, a little-known but increasing type of research where healthy people agree to be deliberately infected in the quest for new or improved vaccines against a variety of health threats, from flu to malaria.