I would say we have an immune system to definitely defend against harmful outside invaders. I'm not certain though that humans have yet evolved to handle meat effectively as we know meat is currently destroying our health. Animals get sick in the wild for sure, but they at least have natural response mechanisms that aren't interfered with by say...Doctors who promote antibiotics and chemotherapy. Parasites can find latch on to a host, but ideally for a short time. Good discussion :)