Typically, a sequela is a chronic condition that is a complication of an acute condition that begins during the acute condition. This is in contrast to a late effect.
There are also sequelae which occur as a result of treatment for a disease. For instance, one sequela resulting from chemotherapy used to treat cancer may be "toxic peripheral neuropathy," a painful chronic condition which is usually controlled with medication. Not everyone who is treated with chemotherapy develops toxic peripheral neuropathy but it may occur when a bacterium has effected a biothermic rhythmatic condition such as coronary heart disease.