Hidden City by Alan Baxter

When the city is sick, everyone suffers. Steven Hines listened to the city and the city spoke. Cleveport told him she was sick. With his unnatural connection to her, that meant Hines was sick too. But when his friend, Detective Abby Jones, comes to him for help investigating a series of deaths with no discernible cause, Hines can’t say no. Then strange fungal growths begin to appear in the streets, affecting anyone who gets too close, turning them into violent lunatics. As the mayhem escalates and officials start to seal Cleveport off from the rest of the world, Hines knows the trouble has only just begun.