August 19th, 2003: A green cloud passed over the server, infecting everyone with a 'Type-A' and 'Type-B' virus. Both would transmit to other players via uNFs. Type-A passed to you if the person you were uNFing had it, or to them if you had it. Type-B only passed when you uNFed a person infected with it. A biohazard symbol displayed and your health was damaged whenever an infection was present while uNFing. Type-B hurt more than Type-A.
The virus spread naturally over the following month and a half, after 25 people were initially infected. By the end of that evening, 44 were infected with Type-A and 32 with Type-B. A week later, 440 had Type-A and 128 had Type-B. When all was said and done on October 10th, 2,589 had Type A, and 307 had Type-B.
Nobody knew what this was all about while it was happening, so the virus spread at a natural rate.