Context: An infectious disease is a disease that is caused by a pathogen. A pathogen is an organism or virus that causes disease in another living thing. Pathogens are commonly called germs. Watch this dramatic video for an historic perspective on infectious diseases and their causes: . MEDIA Click image to the left or use the URL below. URL: There are several types of pathogens that cause diseases in human beings. They include bacteria, viruses, fungi, and protozoa. The different types are described in Table 21.1. The table also lists several diseases caused by each type of pathogen. Many infectious diseases caused by these pathogens can be cured with medicines. For example, antibiotic drugs can cure most diseases caused by bacteria. Different pathogens spread in different ways. Some are easy to catch. Others are much less contagious. Some pathogens spread through food or water. When harmful bacteria contaminate food, they cause foodborne illness, commonly called food poisoning. An example of a pathogen that spreads through water is the protozoan named Giardia lamblia, described in Table 21.1. It causes a disease called giardiasis. Some pathogens spread through sexual contact. In the U.S., the pathogen most commonly spread this way is HPV, or human papillomavirus. It may cause genital warts and certain types of cancer. A vaccine can prevent the spread of this pathogen. Many pathogens spread by droplets in the air. Droplets are released when a person coughs or sneezes, as you can see in Figure 21.2. The droplets may be loaded with pathogens. Other people may get sick if they breathe in the pathogens on the droplets. Viruses that cause colds and flu can spread this way. Other pathogens spread when they are deposited on objects or surfaces. The fungus that causes athletes food spreads this way. For example, you might pick up the fungus from the floor of a public shower. You can also pick up viruses for colds and flu from doorknobs and other commonly touched surfaces. Still other pathogens are spread by vectors. A vector is an organism that carries pathogens from one person or animal to another. Most vectors are insects such as ticks or mosquitoes. They pick up pathogens when they bite an infected animal and then transmit the pathogens to the next animal they bite. Ticks spread the bacteria that cause Lyme disease. Mosquitoes spread the protozoa that cause malaria. What can you do to avoid infectious diseases? Eating well and getting plenty of sleep are a good start. These habits will help keep your immune system healthy. With a healthy immune system, you will be able to fight off many pathogens. Vaccines are available for some infectious diseases. For example, there are vaccines to prevent measles, mumps, whooping cough, and chicken pox. These vaccines are recommended for infants and young children. You can also take the following steps to avoid picking up pathogens or spreading them to others. Watch this video for additional information on preventing the spread of infectious diseases: MEDIA Click image to the left or use the URL below. URL: Wash your hands often with soap and water. Spend at least 20 seconds scrubbing with soap. See Figure 21.3 for effective hand washing tips. Avoid touching your eyes, nose, or mouth with unwashed hands. Avoid close contact with people who are sick. This includes kissing, hugging, shaking hands, and sharing cups or eating utensils. Cover your coughs and sneezes with a tissue or shirt sleeve, not your hands. Disinfect frequently touched surfaces, such as keyboards and doorknobs, especially if someone is sick. Stay home when you are sick. The best way to prevent diseases spread by vectors is to avoid contact with the vectors. For example, you can wear long sleeves and long pants to avoid tick and mosquito bites. Using insect repellent can also reduce your risk of insect bites. 
 Question: Which type of pathogen causes AIDS?
Output: virus