The symptoms of Space Sickness include shortness of breath, nausea, coughing, high fever, and muscle pains. Diarrhea and vomiting can follow. Onset can occur between two and ten days from the point of exposure, and the majority of people with healthy functional immune systems will not catch the disease. Luckily, the disease is rarely passed from person to person, but is most commonly transmitted through contaminated water supplies, ventilation systems, and other areas with aerosol water exposure.
The most common mode of exposure to Space Sickness comes from interplanetary travel. It doesn't take much for a contaminated water supply to potentially infect an entire shuttle of passengers moving from the surface of a planet to an orbital hub. Long duration ships such as military cruisers, passenger liners, or exploration ships are the highest risk. An infected water supply can contaminate hydroponics sectors, air recirculation systems, air filtration systems, and so forth. The longer a passenger is on a ship, their chance of infection rises. Likewise, the age of the ship or habitat also plays a factor, as older facilities are less likely to be held to the same rigorous cleaning and maintenance schedules.
Treating space sickness is a relatively simple affair, and most people can self diagnose the disease through their autoscanner, and have the appropriate medications automatically delivered to them.
On long space runs, Space Sickness can cause the largest problems, where maintenance and extensive cleaning of a ship cannot fully purge the water supply of the contamination, and medical supplies can be limited. This can become aggravated as the symptoms are fairly generic, and can be ignored as a cold or other general respiratory infection. Once the pneumonia aspect is manifested, the disease can strain crew morale and supplies, as well as even kill older and weaker victims.
This danger does remain largely because of the amount of time required for a proper quarantine would be excessively restrictive, no one can afford a ten day stay at each change-over or stop on a trans-system voyage. Space travel is already slow enough as it is without mandating weeks of medical isolation. The present risk of disease, something much more severe for the immuno-suppressed residents of arcologies (glass bubble dwellers), is one of the reasons that virtual commuting is much more popular than analog/actual traveling.