First principles

It has now been proved that for HIV transmission to happen, three conditions need to apply:

• Live virus has to be present either in the body of an infected person or in a contaminated body fluid or body tissue (Presence).
• There has to be a sufficient amount of the virus present (often called technically `tissue culture infectious dose') (Quantity).
• It has to get into the body of the uninfected person through an effective route for transmission to occur. In this respect HIV is no different from any other virus: it can only be transmitted through certain now well–understood routes. There must be susceptible receptor cells at the site of entry and inadequate host defences (Route and Susceptibility).

Additionally, there are two sets of co–factors which may be said to encourage transmission or make it more likely:

• Physical co–factors.
• Social co–factors.

These principles can be used to determine the following:

• Clear guidelines on the established and predominant modes of transmission.
• Clear guidelines on biologically possible but unlikely modes of transmission.
• Clear guidelines on `impossible' modes of transmission and reassurance against unfounded concerns about HIV and AIDS.

All of these factors are now discussed in more detail immediately below.

The grounds for confidence in this knowledge are discussed in The evidence for HIV transmission, later in this chapter.