Infectious diseases are illnesses caused by viruses or bacteria that are spread between people or from animals to people. Most infectious diseases cause mild short term effects, while others can cause more serious long term effects. Researchers on homelessness and infectious disease often focus their investigation on Hepatitis A, B and C, tuberculosis, HIV / AIDS and a range of sexually transmitted infections.
Homelessness has an impact on the spread, severity and treatment of infectious diseases. Homeless people are vulnerable to illness because of their compromised immune system and inadequate nutrition. Inability to maintain personal hygiene, and having to stay in overcrowded and sometimes poorly ventilated shelters and drop-ins along with many other people who are also vulnerable to illness raises the likelihood of the spread of infectious disease. In addition, activities that some homeless people engage in, in order to survive (the sex trade) and intravenous drug use amongst a subset of the homeless population also increases the risk of the spread of disease.
Treatment of infectious diseases amongst the homeless is complicated by the fact that many homeless people face barriers in accessing health services, lack funds for medication, and may not easily be able to follow the routines of treatment. Ultimately, the fact that people who are homeless have few options but to continue to use overcrowded emergency services means that even if they have access to treatment, they are still at high risk for other infectious diseases.