Dengue Virus

 (DENV) is a bacteria that causes dengue fever.

Appearance
Dengue Virus appears as a harmless red mask with a pulling allure. In the anime it seemed like it only infected Langerhans Cells.

Background
Dengue Virus appears as a mask that fell from a wound inflicted by a mosquito. Langerhans Cell picked it up and stated that it seemed very attracting and alluring. Once he put the mask on, the mask girpped on tightly to his face.

Trivia

 * A virion of dengue is depicted as a mask of a Tengu, a spirit or yōkai from Japanese folklore. This is a visual pun on the phonetic similarity between the words "Tengu" and "dengue" (デング) in Japanese.
 * Cells infected by the virus are even granted the gust-blowing abilities of a Tengu's aralia leaf.
 * The Dengue Virus is similar to the Stone Mask from the JoJo's Bizarre Adventure series. The masks have similar functions and behaviors.