  We have probed how the birefringence of a healthy red blood cell (RBC)
changes as it becomes infected by a malarial parasite. By analyzing the
polarization properties of light transmitted through a single,
optically-trapped cell we demarcate two types of birefringence: form
birefringence which depends on the shape of the cell and intrinsic
birefringence which is brought about by the presence of the parasite. We
quantitatively measure changes in the refractive index as normal RBS become
infected by a malarial parasite. Malarial infections are found to induce
changes in the cell's refractive index whose magnitude depends on the stage of
malarial infection; such changes were quantitatively explored and found to be
large, in the range 1.2 to 3$\times10^{-2}$. Our results have implications for
the development and use of non-invasive techniques that seek to quantify
changes in cell properties induced by pathological states accompanying diseases
like malaria. From a broader prespective, information forthcoming from our
trap-based experiments may stimulate physicists to initiate theoretical and
computational efforts that have a bearing on cell biology issues related to
diagnosis of diseases and therapeutics.
