definite article with initialisms

When an initialism refers to an organization, the definite article generally precedes it. For example:

The WWF (World Wide Fund for Nature) is an international conservation body.

However, when the initialism refers to a substance, method or condition, the definite article is not required:

DDT was banned in the United States in 1972. (substance)
St. John Ambulance offers CPR training. (method)
ADD/ADHD is often diagnosed when children begin school. (condition)

Since the initialisms HIV (human immunodeficiency virus) and WNV (West Nile virus) both refer to medical conditions, the definite article is not needed:

WNV was first detected in North America in 1999.
HIV is not spread through casual contact.
Three percent of the population tested HIV-positive.

But:

The HIV-positive population is increasing yearly.

In the last example, the definite article is necessary because it refers to the noun population rather than the initialism.