Jungle Buck by Margaret Madigan

Two months after her second run-in with the Russian mafia, Dr. Melinda Emerson finally admits she’s struggling with PTSD. Her relationships are suffering—especially with her boyfriend Grady “Buck” McCormick, who’s recently moved into her house. She’s just decided to get help when she gets a lead on a village in the Amazon where nobody’s ever had dementia. Ever. After the disastrous experiment that led to the catastrophe in Siberia, Melinda’s determined to refocus her work on a cure for Alzheimer’s, and puts together an expedition to the jungle.

Grady “Buck” McCormick is on leave after almost dying in Siberia. Melinda believes it was her fault, but he’s done everything he can to support her. After all, he’s seen enough vets with PTSD to know what it does to a person. So when she plans a trip to the Amazon, he insists on going along as security detail, not that anything will happen that requires security. He just wants a chance for them to talk and begin healing.

Buck’s hope for relationship-building is snuffed out when Melinda brings along her lab assistants April and Cody. On top of that, when a logging company clearing land for farming threatens to destroy the village, Buck agrees to talk to them only to find an old SEAL buddy in charge. The village elders refuse to leave their homes, Melinda refuses to move her research, and his buddy refuses to back down, leaving Buck stuck in the middle. Things go sideways when his buddy kidnaps Melinda to force Buck to his point of view. Bad idea. Nobody threatens Melinda without paying for it.