Nature Med. 5, 1052– 1056 (1999).

On page 1052, the last two column headings in Table 1 lacked the correct superscripting. The correct column headings are "particles/mla" and A20+capsids/mlb." On page 1053, the column headings for Table 2 were shifted. The correct table is printed below.

We regret these errors.