When talking about diseases, we’re going to have to clarify a bit. Technically a disease is any condition that impairs your normal body functioning. Some diseases are hereditary, which means you are naturally more likely to get it because someone in your family has had it, like hemophilia and sickle-cell anemia, and the gene was passed down to you. Most of these diseases cannot be passed from person to person. If someone who had diabetes coughs on you while you are riding on the train, you will not get diabetes, although you might be angry that they have exposed you to a number of other germs they were carrying. Some diseases are primarily caused by lifestyle choices, such as diet for obesity and high-cholesterol, and smoking for lung cancer. Some diseases like cancer can be caused by exposure to radiation, which is why they put a lead vest over your private regions when you are getting an ex-ray.
Infectious diseases are really the only ones you need to worry about “catching”, and I’m assuming this is the type of disease you are asking about. Infectious diseases, like the flu, are bacteria, fungi, and viruses that can be spread by coming into physical contact with them. For example, imagine I am on the train, and I have the flu. The flu is a virus that takes over my cells, turning them into virus factories that make more viruses until the cell is killed by my immune system, which leave the cell and try to infect more cells. I ignore all the public health advice that EVERYONE should know, unless you still believe that people still get sick because they are possessed by demons, and I let go a giant sneeze that sends atomized bits of snot and saliva all over the car (for the record, you should always sneeze and cough into your sleeve, or a handkerchief, and not on your hands). Inside those bits of snot and saliva will be copies of the flu virus, and wherever they land will have the potential to infect people. If you breath in the “perfume” that I just sprayed out, you could be infected. If you touch a door handle or grab bar that I hit, then touch your eyes or mouth, you could get infected. Once the disease has entered your body, it is up to your own immune system to decide whether you get sick or not.
There are a lot of different ways these diseases can be spread. You can drink water contaminated with sewage, for example, and get e. coli or hepatitis. A mosquito that has already drawn blood can take blood from you, too, and boom! West-Nile Virus. You can get bitten by a komodo dragon, and the bacteria their saliva infects you with could bring you down within a day, making you an easy meal for an animal that can eat 80% of its body weight. Less exotic is athletes foot, a fungus spread by exposure to contaminated gym locker rooms, or the common cold, spread by sneezes and coughs.
The best defense against infectious diseases are good habits and a strong immune system. Washing your hands frequently, brushing your teeth, and being careful about what you eat and drink can greatly limit your exposure to viruses and bacteria. Keeping yourself healthy with a good diet and exercise habits will keep your immune system in good shape to fight off the germs you will inevitably get. And if you are sick, do the right thing and keep your nasal perfume to yourself!
Posted 4661 day ago