Infection

An Viral infection can be caused by either being Bitten injection, or any other way managing it to enter the blood stream. 'Host' is a term for a person or creature infected with the T or G-Virus.

Zombie - human Infection
This form of infection can last upto several hours before transforming the host into a Zombie, however just because it can last several hours doesn't mean it can't last several minutes as seen in Resident Evil: Outbreak.

Leech - Human Infection
Being Leeches, they feed via sucking the blood of creatures, allowing the Leech to infect faster. between the attack on the train and the discovering of the zombies by Rebecca Chambers, is would take less than 2 hours, maybe even minutes. Sometimes the leeches will stay on the body making a 'Leech Man' which will turn into a Zombie within minutes, with the Leeches still hanging on.