The disparity is three logs minimum.
The human genome is massive 2 billion bp, bacteria are 100 million and viruses 10000 bp for RNA viruses. The copy number of the pathogen therefore has to be huge to compensate for the disparity in genome size.
Think about it for a minute if it is 1% then it means for each peripheral mononuclear blood cell (PMBC 'white blood cells') there are 20 million e.g. bacteria ... thats a dying person. The immune system would be incapable of coping. In an infection of that intensity the PMBCs would flood the systemic circulatory system, maintaining a clear imbalance in genome disparity.
You are very unlikely to obtain any DNA viruses abeit they have large genome upto 200Kbp because they are present at low copy number (given smallpox is eradicated).
Where there is a higher chance of success if the individual is pregnant, which causes immunosuppression to protect the foetus or immunocompromised for example HIV, transplantation drugs or chemotherapy amongst others.
The other scenarios you are likely to encounter 'success' involve extremely virulent pathogens such as Ebola (RNA virus), later stages of septicaemia etc..