Keeping a room cold often keeps the humidity in a good range. Heating the air can cause a drop in relative humidity. This can open the entrance path for the bacteria and viruses. The effect is often misinterpreted. We measured the temperature and humidity in our office. We had about 26°C and 10% humidity. People kept catching colds several times in the winter time. As we could not do much about the temperature (high powered PCs provide all the heating we need) we got lots of plants the humidity went up to 30% - 40% and sick leaves dropped sharply.