The green card or permanent resident card is the document that allows immigrants to live, work and make a future in the United States.
Although there are good chances to get it, not all foreigners succeed. This is mainly because, before getting it, they must pass a rigorous medical examination that aims to rule out diseases that are considered very dangerous.
Having one of those diseases will prevent you from obtaining the green card.
Such diseases are:
Syphilis: it is a sexually transmitted disease (STD) that can cause long-term complications or even death if not treated properly. Symptoms in adults are presented in phases. The stages are: 
primary, secondary, latent and advanced syphilis.
Gonorrhea: it is an STD that can infect both men and women. It can cause infections in the genitals, rectum and throat. It is a very common infection, especially in young people aged 15 to 24 years.
Type A Tuberculosis: it is a bacterial infection that usually causes cough, fever, phlegm (sometimes with blood), tiredness and even death. It is considered the second infectious disease with the largest number of affected people after AIDS.
Leprosy: it is an infectious disease, but it is not highly contagious when properly treated. However, patients who are not or improperly treated do constitute a source of infection, because they can develop an insufficient immune response to hold back the infection.
According to immigration officials, people who suffer from these dangerous diseases represent a threat to public health in the United States. For this reason, their residency applications are not approved.
Mental Illness
Those suffering from a serious mental illness that includes harmful behavior will not receive a green card either.
What about HIV?
Since 2010, it is not necessary to have an HIV test taken to apply for a green card. Being seropositive will not prevent you from getting the U.S. residency.
Other Cases
In some cases, after a person who suffers from a disease becomes a legal permanent resident, he or she may be deemed as a public charge. For example, when a person cannot work because of illness or because he or she has a condition that requires expensive drugs for the rest of his or her life.
In very rare cases, the financial sponsor is required to prove they have higher resources than expected or to prove they have health insurance to cover the disease.
To Keep in Mind
Green card applications submitted by drug addicts are always rejected