Colds can cause encephalitis, commonly viral encephalitis, but not all people with colds develop encephalitis, which tends to be a minority of cases. Whether or not a cold causes encephalitis depends on the route of infection, one of which is the bloodstream. There is a natural barrier in the brain tissue that prevents infections from entering the brain tissue, called the blood-brain barrier, and after a person has a cold, as long as the blood-brain barrier functions normally, viruses or bacteria that enter the bloodstream will be blocked from entering the bloodstream and will not be able to cause encephalitis if the blood-brain barrier is not functioning properly. However, if the patient has previously suffered from hypoxia or trauma, resulting in the destruction of the blood-brain barrier, then once a person has a cold, pathogens will enter the blood-brain barrier to reach the brain tissue, causing encephalitis. There is another pathway that is less common, a special structure within the brain tissue called the trigeminal semilunar ganglion. In the process of growing up, usually have been exposed to herpes simplex virus, these herpes viruses will be latent in the trigeminal semilunar ganglion, after the cold body resistance decline, these viruses will be replicated in large quantities, and along the trigeminal semilunar ganglion backward retrograde into the brain tissue, triggering herpes simplex encephalitis. Therefore, if you have a cold, do not take the cold symptoms lightly, and should be treated in time. If you have a persistent headache, fever, or even nausea and vomiting after a cold, you should be alert to the symptoms of encephalitis, and you should go to the hospital in time for timely treatment.