How long can plague bacteria exist in the environment?
Yersinia pestis is easily destroyed by sunlight and drying. Even so, when released into air, the bacterium will survive for up to one hour, depending on conditions.
What is cholera?
How does a person get cholera?
A person may get cholera by drinking water or eating food contaminated with the cholera bacterium. In an epidemic, the source of the contamination is usually the feces of an infected person. The disease can spread rapidly in areas with inadequate treatment of sewage and drinking water.
The cholera bacterium may also live in the environment in brackish rivers and coastal waters. Shellfish eaten raw have been a source of cholera, and a few persons in the United States have contracted cholera after eating raw or undercooked shellfish from the Gulf of Mexico. The disease is not likely to spread directly from one person to another; therefore, casual contact with an infected person is not a risk for becoming ill.
If smallpox is released in aerosol form, how long does the virus survive?
The smallpox virus is fragile. In laboratory experiments, 90% of aerosolized smallpox virus dies within 24 hours; in the presence of ultraviolet (UV) light, this percentage would be even greater. If an aerosol release of smallpox occurs, 90% of virus matter will be inactivated or dissipated in about 24 hours.
So, to summarize all that, there should be no risk from contact with any corpse more than a day old. The plague can last for up to an hour, cholera doesn’t spread from person to person, except through fecal contact, and aerosolized smallpox dies within 24 hours. I assume that non-weaponized smallpox wouldn’t last as long or be as contagious. The same site states that extensive face-to-face contact would be needed to spread the disease.
Note: all info is from the CDC.