The cacti find that they need help, once the humans have discovered their vampirism and cut them down and burn them in huge bonfires that snuff out the daylight with clouds of black smoke. They call to the smaller, smarter variety, the common cactus houseplant to help them. Cacti in Walmart, in people's homes, in restaurants, in public gardens all hear the call. While not infected with vampiric blood, the ever thirsty soul of cacti is awakened by the call and they begin to hunt at night, moving ever slowly closer to the humans who care for them. Sometimes leaving their prickles in shoes, or under pillows, or on the toilet seat. Wherever a human might accidently get poked and suffer.
This is the second kind of death, a long torturous draining of fluid. At first the effects are not noticeable and the person might just wake up with chapped lips and a dry throat which no amount of carmex or water can alleviate. Then the skin on legs and arms dries and cracks, sloughing away like onion peel. Vision becomes blurry and the human is more and more fatigued, until unable to get out of bed. Cacti gather around the prey to suck the last bit of moisture out, until the person becomes a dry husk, a tumbleweed in human rags.