--run the ELISA test first for a number of infectious conditions, and the run a Western Blot if there are suspicious results from the ELISA. This is mainly due to cost concerns more than anything else.

The ELISA (enzyme-linked immunoabsorbent assay) is a subtype of the overall EIA (enzyme linked immunoassay) that is generally used on liquid samples (such as serum).

I prefer the Western Blot myself, as it is more sensitive for a lot of antigens.

There is also the possibility of using a PCR (polymerase chain reaction) test to more directly test for the presence of an infectious agent, but this involves an attempt to culture the agent and takes more time (and is therefore even more expensive).