Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
An image that was published alongside another PLoS Medicine article (doi/10.1371/journal.pmed.0020024) and included in the shared PDF download here has been removed from the published PDF because of that image's copyright status.
No competing interests declared.
View on publisher site
