Answer: Communicable diseases are most often spread by microrganisms which are transferred from individual to individual by droplet contact (sneezing, kissing, etc.), sexual (bodily fluid) contact, foodborne contact, and water source contact. Sometimes skin to skin contact is sufficient, as well.
Answer: Yes, they can for example bubonic plague carried by rats can be spread by fleas taking blood from the rat then hopping onto another animal or person thereby infecting them. A direct bite can do the same thing.
Answer: Diseases can be spread ten ways that I know of:
1. Through the air,i.e.by people coughing, sneezing or breathing on you. This is called droplet infection since their breath contains tiny water droplets, containing the harmful microbe.e.g.the flu and influenza.
2. Eating contaminated food, such as chickeni.e.food poisoning.
3. Drinking contaminated water. For example, if sewage has gotten into your drinking water you could catch Salmonella andE-Coli.
4. During sexual intercourse diseases can be passed from one person to another,i.e.syphilis (bacterial); yeast; HPV (viral).
5. Through a cut. For example, if you punctured your foot by stepping on a rusty nail, you could catch Tetanus which is fatal if untreated.
6. From mother to baby through the placenta,i.e.Rubella.
7. Through blood. For example, drug addicts sharing needles could catch HIV. (Human Immunodeficiency virus) Although the USA blood donor supply is scrutinized through massivepre-testing,recipients of donor blood can contact hepatitis, HIV, etc.
8. By touching people.e.g.fungus or bacterial infections. Or by stepping barefoot in public shower areas, i.e. Athletes Foot which is caused by a fungus.
9. Animal bites can spread diseases. for example mosquitoes can carry malaria and dogs can carry rabies.
10. Depressed auto-immune systems or an existing condition which makes a person susceptible to many infections. For example, diabetes can lead to urinary infections and skin or vaginal yeast infections.