1. In dense equatorial forests,which have not been degraded (where Man is practically absent), the transmission of malaria is very low or non-existent. This can be explained in two ways: the major vectors of malaria are not very numerous, the dense forest is obviously unsuitable for them; furthermore, Man, with the exception of Pigmies, is practically absent from this area. There is therefore no reservoir of parasites. Transmission is poor, around 10 PAIH (bites of anophelines infected by Man) per annum. 

2. In degraded forests,along a river or a road, the situation is radically different. Deforestation, natural or anthropogenic, has a strong impact on the transmission of malaria. The sun’s rays penetrate right into the larval breeding grounds. Henceforth, these breeding grounds become favorable to the development of Anopheles gambiae, which may become very abundant. The conditions of survival (temperature, hygrometry) being normally favorable for the survival of the adult, the transmission is very high, frequently in the region of several hundred PAIH per annum. Transmission is permanent throughout the year. 

3. In savannas,  on the contrary, the seasonal variations are considerable. Transmission takes place solely during the rainy season and at the beginning of the dry season. It may be considerable, varying between several hundreds and tens of PAIH depending on the locality. Frequently, a succession of vector roles between Anopheles gambiae  and/or Anopheles arabiensis may be observed in the middle of the rainy season, then Anopheles funestus at the beginning of the dry season.  

4. In the Sahel,the climatic conditions are usually too harsh to allow the transmission beyond two months in the rainy season. Locally, especially around lakes, the transmission period may be extended. The transmission rate is around one to a few dozen PAIH per annum. 

5. In the Sahara,the transmission of malaria is an abnormal event, and furthermore relatively poorly documented. Its effect on the epidemic mode is only to be found in the most favorable locations, i.e. oases, eventually leading to an increase in reservoir of parasites following the arrival of malaria-infected immigrants. 

6. In irrigated areas,ordinarily linked to hydro-agricultural developments (for example rice paddies in the savanna areas), environmental modifications give rise to very variable repercussions. In all cases, there is an increase in the density of malaria vectors. But there is not always an increase in the transmission rate: that depends upon the base transmission level prior to the introduction of these developments. When this base level is low, it is not rare to see an increase in the transmission of malaria. On the contrary, when the base level is already high, the consequences of the developments on the transmission rate are low or non-existent.  

7. In town,the transmission of malaria is a perfectly perceptible reality. It is ordinarily around 0.1 to 3 PAIH per annum. These rates are the lowest observed in tropical Africa. Urbanization alone is responsible for this upheaval in the epidemiology of malaria. A transmission gradient ordinarily observed from the most peripheral recent districts, where the transmission is hardly lower than the neighboring rural area, towards the older and central districts, where the transmission is much lower. Nowadays, it is no longer more exceptional to encounter adult men who have always lived in an urban environment and have escaped, by chance, any infection by malaria. Without malarial experience, they are in an immunological state close to that of a traveler coming from a country free of malaria. It is foreseeable that this embryonic situation may develop with increasing urbanization, as much in absolute values as in percentage of the population concerned, and that these potentially epidemic situations should be considered as average and long term. 

8. It is interesting to note that it is the value of the transmission rate which determines the epidemiological context. In fact, by definition, malaria is called  ENDEMIC when its transmission rate is more or less constant and perceptible for several consecutive years. On the contrary, it is called EPIDEMIC when its transmission rate is unusual, either due its intensity during an exceptional period, or by its location in an area where it is normally not prevalent.