Even though Talona is the goddess of illness, poison, and plague, she's also the goddess of medicine and healing. Every vaccine needs a bit of the virus to work and all antivenoms are derived from the venom which they prevent. She constantly goes through cycles of sickness and health, leaving people to be healthy when they worship her and bringing plagues when they forget about her.
As a priest of Talona, you're downright terrified of your goddess. She could inflict a myriad of ailments on you at any time if you are dissatisfactory to her plans, so you do everything in your power to please her.
Your ultimate goal is to have her widely worshipped everywhere alongside other popular members of the pantheon. So long as she's appeased, she can be a kind goddess but anywhere that forgets her blessing will receive her wrath.
Preach to others that she isn't a god who will promise a safe or glorious afterlife, but her presence is felt in the living world. Even in times of perfect health, she's still there, just her more inconspicuous and kinder form. It's easy to forget her blessings, see health as the status quo, and see her as purely evil, but she's merely angry at mortals for not showing any gratitude to her like they do to other gods.
At times she sends tough-love reminders, a bit of food poisoning here, a minor cold there. Nothing that would kill you, but enough to make you cherish the times of health. But if left unworshipped or worse, cursing the goddess for the illness, then that's when she brings full on epidemics as lessons of who's in charge.
If they just show praise to the Mother-of-All-Plagues, then they will continue to feel her love. But if they disrespect her name and fail to give her praise, then you're not responsible for the horrors of the flesh she will inflict.