The increasing use of antibiotics contributes to the development of drug-resistant bacteria, which can then spread from person to person via contact with their feces or urine. Resistance can also develop as a result of direct exposure to antibiotics. Bacteria exposed to high levels of these drugs will likely be resistant to them. Even low doses of antibiotics can lead to the development of resistance if patients do not fully recover from their infections but rather remain carriers of the bacteria that cause them. Carriers go on to infect other people who are then treated with antibiotics, contributing further to the evolution of resistance. Finally, viruses can also evolve ways to avoid being killed by antibiotics. Some viruses adapt easily to their host's immune system by changing themselves or their structure. This can happen when a virus encounters something similar to itself in another species. For example, someone infected with HIV might be given immunosupressants to reduce the risk of their infection progressing to AIDS. This causes many cancers associated with HIV infection to grow more quickly because the body no longer has the resources it needs to fight them. Viruses can also adapt by moving directly from one species to another.