Is there a minimum load below which an infectious agent will not cause disease?
It depends on the pathogen, delivery method, environment, etc. In most cases, a single pathogen is not enough to cause disease -- it may need many millions or billions -- but there are some cases where it does seem that a single pathogen can consistently cause infection. For example


  The minimum infectious dose of ASFV in liquid was 100 [i.e 1] 50% tissue culture infectious dose (TCID50), compared with 104 TCID50 in feed. The median infectious dose was 101.0 TCID50 for liquid and 106.8 TCID50 for feed. 


--Infectious Dose of African Swine Fever Virus When Consumed Naturally in Liquid or Feed

If you're asking specifically about smallpox, I don't think anyone has done the experiment (the natural and most susceptible host being humans, I think you'd have some trouble finding volunteers to find to lethal dose). A single virus of the related ectromelia (mouse pox) virus can cause death in particularly susceptible mouse strains:


  The intranasal lethal mousepox model employing the A/Ncr mouse strain is used to evaluate anti-orthopoxvirus therapies. These infections mimic large droplet transmission and result in 100% mortality within 7-10 days with as little as 1 PFU [plaque-forming unit] of ectromelia virus. 


--Mousepox in the C57BL/6 strain provides an improved model for evaluating anti-poxvirus therapies

As a side note, the notion of what a "single virus" actually is, is more fuzzy than you might think, but that's a more complicated question; to a rough approximation a single TCID50 or PFU can be considered to be a single virus.