Bathrooms gather a lot of moisture and therefore they need ventilation. Many states legally require that bathrooms have some sort of ventilation because it can prevent water damage in a house. There are methods of bathroom ventilation that will work with any home and bathroom.
A recirculating fan can easily be installed into a room. These fans suck the moist air up and remove most of the moisture before blowing it back into the room. While not all of the moisture gets taken out this way, it does eliminate some of it. One consideration when using this method is that these fans are not connected to any outside source so fresh air is not brought into the room.
There are two types of natural ventilation. One is simply a window. This is a viable ventilation option if the bathroom is built on an outer wall. If you live in an area with low temperatures during the winter then this may not be the best option because it will occasionally be too cold to open the window. Also, if the window is not high enough people can potentially look through it from the outside. The other natural ventilation option is to have a fan and duct that goes through the walls and filters the air to the outside. These do not let cold air or other outdoor elements inside. However, there is a chance that there could be water damage to the house if the duct is not installed properly.
For rooms that are not near an exterior wall, some homeowners opt for exhaust fans. These fans filter the air in the bathroom to rooms outside, either up towards the roof or out to the room on the other side of the wall. This filtration allows fresh air to come back into the bathroom. Exhaust fans are not difficult to install, but the noise factor is a problem for some people. Some fans are louder than others, so if the quiet fans are not in your budget make sure you are OK with the sound level of the fan.