16 Large-capacity dormitories inevitably imply a lack of privacy for prisoners in their everyday lives. Moreover, the risk of intimidation and violence is high. Such accommodation arrangements are prone to foster the development of offender subcultures and to facilitate the maintenance of the cohesion of criminal organisations. They can also render proper staff control extremely difficult, if not impossible; more specifically, in case of prison disturbances, outside interventions involving the use of considerable force are difficult to avoid. With such accommodation, the appropriate allocation of individual prisoners, based on a case by case risk and needs assessment, also becomes an almost impossible exercise. All these problems are exacerbated when the numbers held go beyond a reasonable occupancy level; further, in such a situation the excessive burden on communal facilities such as washbasins or lavatories and the insufficient ventilation for so many persons will often lead to deplorable conditions. The CPT must nevertheless stress that moves away from large-capacity dormitories towards smaller living units have to be accompanied by measures to ensure that prisoners spend a reasonable part of the day engaged in purposeful activities of a varied nature outside their living unit. Access to natural light and fresh air 30. The CPT frequently encounters devices, such as metal shutters, slats, or plates fitted to cell windows, which deprive prisoners of access to natural light and prevent fresh air from entering the accommodation. They are a particularly common feature of establishments holding pre-trial prisoners. The CPT fully accepts that specific security measures designed to prevent the risk of collusion and/or criminal activities may well be required in respect of certain prisoners. However, the imposition of measures of this kind should be the exception rather than the rule. This implies that the relevant authorities must examine the case of each prisoner in order to ascertain whether specific security measures are really justified in his/her case. Further, even when such measures are required, they should never involve depriving the prisoners concerned of natural light and fresh air. The latter are basic elements of life which every prisoner is entitled to enjoy; moreover, the absence of these elements generates conditions favourable to the spread of diseases and in particular tuberculosis. CPT: 11TH GENERAL REPORT The CPT recognises that the delivery of decent living conditions in penitentiary establishments can be very costly and improvements are hampered in many countries by lack of funds. However, removing devices blocking the windows of prisoner accommodation (and fitting, in those exceptional cases where this is necessary, alternative security devices of an appropriate design) should not involve considerable investment and, at the same time, would be of great benefit for all concerned. Transmissible diseases 31. The spread of transmissible diseases and, in particular, of tuberculosis, hepatitis and HIV/AIDS has become a major public health concern in a number of European countries. Although affecting the population at large, these diseases have emerged as a dramatic problem in certain prison systems. In this connection the CPT has, on a number of occasions, been obliged to express serious concerns about the inadequacy of the measures taken to tackle this problem. Further, material conditions under which prisoners are held have often been found to be such that they can only favour the spread of these diseases. The CPT is aware that in periods of economic difficulties - such as those encountered today in many countries visited by the CPT - sacrifices have to be made, including in penitentiary establishments. However, regardless of the difficulties faced at any given time, the act of depriving a person of his liberty always entails a duty of care which calls for effective methods of prevention, screening, and treatment. Compliance with this duty by public authorities is all the more important when it is a question of care required to treat life-threatening diseases. The use of up-to date methods for screening, the regular supply of medication and related materials, the availability of staff ensuring that prisoners take the prescribed medicines in the right doses and at the right intervals, and the provision when appropriate of special diets, constitute essential elements of an effective strategy to combat the above-mentioned diseases and to provide appropriate care to the prisoners concerned. Similarly, material conditions in accommodation for prisoners with transmissible diseases must be conducive to the improvement of their health; in addition to natural light and good ventilation, there must be satisfactory hygiene as well as an absence of overcrowding.