Outcome of repeated, low-dose rectal challenges with SIV/DeltaB670. Indian-origin rhesus macaques received weekly low dose rectal challenges. Animals were challenged with 1cc of culture supernatant containing 250 TCID50 SIV/DeltaB670 for the first challenge and 2500 TCID50 for the remaining 5 challenges. Monkey 705 received a 7th challenge with 2.5 ×105 TCID50 72 days after the 6th challenge. Blood was collected twice a week and copies of viral RNA were quantified in plasma by qRT-PCR using external standards. Animals were divided into two groups based on the number of challenges required for systemic infection: highly susceptible (red); 2–3 challenges (R700, R701, R702, R703, R704) and poorly susceptible (blue); ≥6 challenges (R697, R698, R705). Red dotted line at 104 viral RNA copies indicates the threshold virus load (VL) associated with clinical disease in rhesus macaques infected with SIV/DeltaB670. Arrows indicate time points of challenge. Χ = sacrifice for tissue collection.