Though classifications vary across countries, homelessness happens because people cannot access the housing and supports they need. This can be the result of economic and socio-structural factors, like shortage of affordable housing, extreme poverty, and discrimination. It can occur when systems of care and support fail. It can occur in response to individual or relational factors, such as relationship violence or personal trauma. The immediate cause is often an exogenous shock, such as a health crisis, unexpected lack of employment, or abrupt housing loss. But socio-structural factors make certain people especially vulnerable, and gaps in the social safety net and homelessness services systems can extend homelessness or make it more difficult to remain housed.