Tuberculosis (TB) is a chronic infectious disease affecting a large number of people in the developing countries. An on-site, affordable diagnostic screening at an early-stage for an immediate anti-TB treatment is known to tremendously minimize the high mortality rates. Lipoarabinomannan (LAM), a surface glycolipid, has been identified as a potential TB biomarker present in urine at ultra-low concentrations of a few fg/mL. Here, we report a plasmonic fiber optic absorbance biosensor (P-FAB) strategy for mannosylated LAM (Man-LAM or Mtb LMA) detection down to attomolar concentrations. It involves a plasmonic sandwich immunoassay on a U-bent fiber optic probe with gold plasmonic (AuNP) labels functionalized with anti-Mtb LAM immunoglobulin M (IgM) and anti-Mtb LAM IgG respectively. The Mtb LAM is quantified in terms of absorption of light passing through the fiber probe using a green LED and a photodetector. The choice of fiber optic probes (fused silica versus polymer), the optimum size (20, 40, 60 and 80 nm) and concentration (2x, 10x, and 20x) of AuNP labels were investigated to obtain high sensitivity and lower limits of analyte detection (LoD). P-FAB with a simple LED-photodetector pair, 200 mm fused silica U-bent fiber probe and 60 nm (20x) AuNP labels gave LoDs down to 1 fg/mL and 10 fg/mL in the buffer and synthetic urine respectively. Moreover, the anti-Mtb LAM IgM bound sensor probes and the AuNP reagent stored at 4˚C were stable up to 45 days. P-FAB based manLAM sensor demonstrates its potential for an on-site TB diagnosis. Furthermore, the P-FAB immunoassay strategy is explored to detect the SARS-CoV-2 N protein to realize an antigen based SARS-CoV-2 diagnosis.Currently with the optimized P-FAB strategy a detection limit of about 0.37 ng/mL is achieved in the PBS buffer. Work is in progress to realize analyte detection limit down to fg/mL of SARS-CoV-2 N-protein to establish a reliable diagnosis of SARS-CoV-2 infection at an early stage.