Health Department of WA: Chlamydia email
To demonstrate the hidden nature of the Chlamydia virus, an email was send to every person on the Department of Health's database that fell into target audience (males and females aged 15 to 25). Recipients opened the email and saw nothing but a single line of text that instructed them to "Click here and drag to the top of the page."
Doing so revealed a message written in a white, which was previously invisible. It delivered a simple, relevant and stark warning about Chlamydia, and also directed the reader to a website to seek further information and arrange a test.