An infection is easier to avoid than it is to throw off once you've aquired it. To reflect this, you need only make the usual HT+3 check to keep from being infected by a dirty wound, modified by circumstances given on p B444. However, once you have an infection it takes a HT-2 roll to throw it off, unmodified by the conditions listed on p B444. Roll every day.
Most forms of infection cause death through shock. Shock is what happens to your body when blood cannot get to vital organs. Severe bacterial infections raging in your blood cause the peripheral vessels to dialate. As a result, blood pools in the extremities and is not available for normal vital functions. If you are using the semi-cumulative wounding system, a failed HT-2 roll means you take 2 points of Fatigue instead of HP damage, to reflect the similarity to shock from blood loss. This fatigue cannot be recovered until you have thrown off the infection. After that, you can recover FP at the same rate as FP lost from bleeding, with all the usual modifiers for medical care.