Synthetic biology is transforming diagnostic medicine by combining the principles of engineering with biological entities to produce fast, accurate, and manufacturable solutions for global diseases. This chapter discusses advanced applications of synthetic biology-based diagnostics in the form of innovative platforms like CRISPR-Cas9 gene editing, RNA interference (RNAi), and engineered prokaryotic and eukaryotic biosensors. These devices are revolutionizing the detection of bacterial and viral infections, such as sepsis, a potentially fatal condition where early detection is paramount but still evasive with traditional techniques. We emphasize the convergence of synthetic gene circuits, phage diagnostics, and cell-free systems for real-time point-of-care diagnosis, bypassing constraints of conventional culture-based methods. The chapter also discusses limitations in biomarker sensitivity, scale-up manufacturing, and regulatory governance, while presenting breakthroughs such as melanin photothermal imaging and CAR-T cell therapies. One of the central topics is sepsis treatment, where synthetic biology provides customized diagnostics and immune-modulating treatments to reverse dysregulated host responses. The use of cancer examples highlights the cross-disease applicability of these tools, ranging from malignancy to infection. In the future, the intersection of synthetic biology with AI, nanotechnology, and omics will democratize diagnostics, bringing about an age of personalized, affordable, and revolutionary healthcare solutions.