According to a dictionary definition an intercurrent is something that occurs” during or in between” two events. In medical terms it applies to a “pathology occurring during the course of another disease” . This appears to be a very simple definition but in application of homoeopathic treatment the issue is slightly more complex.
Koehler  in “The Handbook of Homoeopathy” describes an intercurrent as an intermediary drug used for a patient where the best selected drug does not achieve a permanent cure. Yasgur  points to its ability of providing motion in a halted case, while Farrington  pronounces its efficacy in bringing the indications for a follow-up prescription to the foreground, and restricts its usage to times when the patient has only shown little reaction to the initial prescription.
So, one could say an intercurrent is a tool to apply if one seeks to identify the underlying aspects of a disease, that have not been touched by the previous, well-chosen homoeopathic prescription.
Hahnemann  mentions in Aphorism 234 of the Organon that in non-febrile morbid conditions and recurrent fevers homoeopathic China has been a useful intercurrent. Opium also served well for this purpose.
In the `Encyclopedia of remedy relationships in homoeopathy’, Rehman  describes intercurrents as “reaction-remedies”, coming into effect if there was no, or little action of the initially administered remedy. He also states their use as interim remedies between two adversary or incompatible remedies. Such remedies should not follow up on one another, and therefore require an intermediary prescription. This may be the case for example, where Hepar sulf. is to follow onto Mercurius sol. or Silicea, or where Nux vom. is needed to complement Causticum or Phosphorus. Rehman  also praises the application of the intestinal-nosodes as Intercurrents for the purpose of unravelling a case. He states that these nosodes make the previously administered remedy spur into action. The intestinal-nosode itself has no effect other than the unravelling of the symptom-jumble.
The use of intercurrents has to be differentiated further, as there are acute and chronic applications. Schepper  points to Aphorisms 36 to 40 for the use of intercurrents in an acute case. Here the intercurrent remedy is used to to treat the acute crisis of a case. If a chronic prescription were made in such a case, the symptom picture of the patient could be altered and undesirable aggravations could be the consequence. Therefore an acute intercurrent in a chronic case is selected by only taking into account the prominent acute symptoms and not the deeper chronic case history. As such, the acute intercurrent is a superficial agent that is used to remove the influence of the acute disease, without having an impact on the constitutional treatment of the chronic case. Hahnemann  describes this in Aphorism 221 of the Organon: The constitutional/chronic treatment has to stop when there is an acute crisis. Once this acute intermediate state is over, the chronic one will continue.
According to Little  in the chronic case an intercurrent is frequently necessary where miasmatic interference is present in the case. Here the symptom picture may be unclear, or symptoms may be scarce. Once an intercurrent has removed the miasmatic layer, symptoms may show more clearly and an appropriate constitutional remedy selection may be possible. Koehler  describes this with relation to the remedy Tuberculinum. As an intermediate remedy, Tuberculinum is indicated where symptoms remain after the first prescription, and the symptom-totality expresses a specific miasmatic taint.
 The free dictionary(2012) Intercurrent last accessed 15 May 2012 at URL http://www.thefreedictionary.com
 Koehler (1989) The Handbook of Homeopathy Vermont, Healing arts press.
 Yasgur, J. (2003) Homeopathic Dictionary New Delhi, B. Jain Publishers (P) Ltd
 Farrington, H. (n.d.) Homeopathy and homeopathic prescribing New Delhi, B.Jain Publishers (P) Ltd
 Hahnemann, S. (1974) Organon der Heilkunst (2.Auflage) 6B Heidelberg:Karl F. Haug Verlag.
 Rehmann, A. (2000) Handbuch der homöopathischen Arzneibeziehungen Heidelberg, Karl F. Haug Verlag
 Schepper, Luc de (n.d.) Using Acute Intercurrent or Intermediate Remedies in Chronic Diseases last accessed 15 May 2012 at URL http://www.drluc.com
 Little, D. (2007) Part 2: The chronic Intercurrents last accessed 15 May 2012 at URL http://www.simillimum.com