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focus, pl. foci (fo´kus, fo´sI)

1. (F) The point at which the light rays meet after passing through a convex lens. 2. The center, or the starting point, of a disease process. [L. a hearth]
conjugate foci two points so related to a lens or concave mirror that an image at one point is focused at the other, and vice versa.
Ghon's f. Ghon's tubercle
natural f. of infection an ecosystem in which an infectious agent normally persists in nature; e.g., yellow fever virus in a jungle monkey-Haemagogus mosquito ecosystem.
principal f. the real or virtual meeting point of rays passing into a lens parallel to its axis.
real f. the point of meeting of convergent rays.
virtual f. the point from which divergent rays seem to proceed, or that at which they would meet if prolonged backward.


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