They share that Lyme “can swim through thicker, more viscous fluids” than syphilis, making it more insidious.
“Syphilis and Lyme disease are better at penetrating our bodies than almost any other organisms. Spirochetes cross barriers that are impenetrable to almost anything else, including basement membranes and the linings of organs like intestines called endothelium that function to keep the kajillions of bacteria in your gut out of the rest of your body. In humans, syphilis and Lyme disease bacteria easily penetrate the normally sacrosanct blood-brain barrier to infect the central nervous system. Syphilis [and Lyme] can invade the placenta and infect an unborn child.
This extraordinary ability is reflected in the symptoms of these brutal diseases.”
Scientific American: On the Curious Motions of Syphilis and Lyme Disease Bacteria