In this chapter, we present the optical diagnosis of normal and dengue viral‐infected human blood using Raman, Polarimetric, Transmission, and Fluorescence Spectroscopic techniques. The possibility of using light in diagnosis and treating illness has been known for thousands of years. The properties of light and lasers provided many modern applications at home, in industry, and in the field of medicine. Laser use in the field of medicine is large and steadily growing. This growth is based on the versatility of laser light. Efficient and accurate diagnosis of dengue is of primary importance for clinical care. A range of laboratory diagnostic methods has been developed to support patient management and disease control. The choice of diagnostic method depends on the purpose for which the testing is done, the type of laboratory facilities and technical expertise available, costs, and the time of sample collection. The dengue viral infection is mostly diagnosed through laboratory tests; these tests include detection of the virus, virus antigen, anti‐dengue virus antibody, complement fixation test, neutralization tests, and detection of virus nucleic acid. As dengue infection most rapidly increases in different regions, early diagnostic confirmation of dengue infection in patients allows for timely clinical intervention, etiological investigation, and disease control. Hence, diagnosis of dengue disease during the acute phase should be a priority and is a public health concern. Lasers and optics have many applications in medical sciences; diagnosis and treatment of diseases with lasers and light are latest and noninvasive techniques. Development of light‐based apparatus has evolved into tools for improved diagnosis and treatment modalities in medical sciences. The methods of the laser spectroscopy make it possible to obtain direct information regarding the structure and dynamics of the functional groups of biomolecules. Development of new light sources, optics, and diode laser of different wavelengths makes them attractive for spectroscopy of biological molecules. In our study, more than 600 dengue viral‐infected blood or blood sera samples and 25 non‐dengue healthy blood samples were analyzed using four different optical methodologies. In the first study, Raman spectrum peaks for normal samples observed at 1527, 1170, and 1021 cm−1 show the presence of different biological materials, including lipids, carbohydrates, skeletal C–C stretch of acyl chains, and guanine. Raman peaks at 1467, 1316, 1083, and 860 cm−1 were observed in dengue‐infected patients, representing CH2/CH3 deformation of lipids and collagen, guanine, lipids, and protein peaks using 532 nm laser sources. In our second study, an optical diagnosis of dengue virus infection in the whole blood is presented utilizing Mueller matrix polarimetry. Mueller matrices were extracted using light source from 500 to 700 nm with scanning step of 10 nm. Polar decomposition of the Mueller matrices for all the blood samples was performed that yielded polarization properties including depolarization, diattenuation, degree of polarization, retardance and optical activity, out of which, depolarization index clusters up the diseased and healthy into different groups. The average depolarized light in the case of dengue infection in the whole blood decreases, whereas for the healthy blood samples it increased. This suggests that the depolarization index of the polarized light was at wavelengths 500–700 nm; in this case, we find that depolarization index values are higher for dengue viral infection when compared to normal samples. This technique can effectively be used for the characterization of the dengue virus infected at an early stage of the disease. In the third experiment, the transmission absorption spectra of dengue‐infected whole blood samples were observed in ultraviolet to near infrared range (400–800 nm) of about 30 conformed infected patients and were compared to normal blood samples. Transmitted spectra of dengue‐infected blood showed two strong spectrum peaks at 540 and 580 nm wavelength of illuminating light, whereas in case of normal blood below 600 nm total attenuation was observed. The two strong absorption peaks from 500 to 600 nm are characteristic of cell damage and dengue virus antibodies IgG and IgM produced against dengue antigen. In the last study, we report an optical diagnosis of dengue‐infected whole blood and controlled samples with Laser Scanning Confocal Microscopy (LSCM) over a laser excitation of 488, 543, and 633 nm wavelength. Based on our findings, the system has potential applications in the detection and quantification of dengue virus‐infected cells, antigen, and antibodies in blood in vitro.