The most common spread of vampirism is a direct bite from a Vampire, the effects of which can be ghastly;

Vampirism outbreaks were an ugly sight on the battlefield, you remember. The King's Guard turned on one another and spread the infection while the Dragul cut their way through your ranks. You can only imagine what Undercity looks like tonight, with its encampments of outcasts and thieves clawing at each other’s necks. - Taron's Trophy (RPA-ADV2)

Infection can also spread by drinking the blood of a Vampire or one of the infected.

Anyone, from any of the races of Ulos, can become infected and make the transformation. In rare instances, animals have also been known to become infected.

Someone is not a true “Vampire” immediately upon becoming infected. Over the course of 1000 years, they will begin to fully transform and develop their powers.

“You will know the infected by their cold gray eyes, long fangs, and the bite marks on their necks.” - Zalic, Outbreak in Undercity (RPA-ADV3)

The initial sensation of a bite turns from pain to power;

His fangs sink deep into your neck, stinging at first but then soothing.

Fangs extend from your gums, and a deep hunger wells up within you. - Outbreak in Undercity (RPA-ADV3)

When they infect one of their victims, a full Vampire can usually manipulate the infected in such a way that they become their thrall, obeying any and all of their commands. Vampires do not always seek to control their victims in this way, however, and infection can spread from those who are not yet full Vampires. When this happens, the infected are not powerful enough to take control of the minds of their victims.

There is no known cure for Vampirism, other than death.

It is rumored that an outbreak of vampirism has occurred in Undercity. You are tasked with seeking out and killing the Vampire and any infected that you may encounter. The truth soon presents itself. You encounter a number of wealthy nobles who purposely are attempting to become infected. If you are not careful, you may find yourself a victim of the infection. - Outbreak in Undercity (RPA-ADV3)